June 1953
1
INDUSTRIAL AND ENGINEERING CHEMISTRY
70% gave additional information. I n the case of samples marked “unsatisfactory,” reasons were frequently given and suggestions offered which have proved very helpful in subsequent work. I n the case of samples for which evaluation work was reported under way, over 60% of the replies gave some description of the work. That every item on the form has been utilized in the replies indicates that the form carries little dead weight. Tables I1 and I11 show a breakdown of answers received to Forms A and B on a percentage basis. This gives a good indication of the type of information which is obtained through the use of the check form and emphasizes the “extra” details t h a t may be acquired. I n any of the steps FOLLOWING SAMPLES TO COMPLETION. along the way in this system, the sample may be taken off the assembly line and given special personal attention. By far the largest majority, however, are pursued t o a positive report by mail. An analysis of these final reports, based on information from all three forms, indicating the disposition of the samples which are sent out, shows: Not reported and assumed dead, % Not evaluated % Unsatisfactory’ for purpose requested, % ’ Satisfactory, %
41.5 17.5 33 8
These figures indicate that approximately 59% of the samples sent out have not stimulated sufficient interest to justify further extensive efforts on the part of the Development Department. The 41.5% of the samples not reported and assumed dead demonstrate a lack of understanding on the part of the prospect of the value of such reports in helping a development department to
1297
satisfy his needs. This problem can be overcome only by education of the individuals and companies who receive samples and fail to make reports on them. Replies covering the 33% of the samples reported to be unsatisfactory require careful study t o determine which justify personal calls. Most of the last 8% of the samples reported to be satisfactory will probably justify the time and expense required to make a call. It should not be assumed, of course, that sales will result on all satisfactory samples. I n many caEes, although the product is satisfactory, i t offers no special inducement to justify a change; that is, of course, a sales problem and not a development problem. Although the preceding figures may be expected t o vary somewhat from product to product, they probably represent an average picture of what happens to free samples which are advertised and offered, since they are based upon results obtained from a variety of chemical products. SUMMARY
The company’s situation with respect to following up samples by mail using the procedure outlined may be summarized by saying that it has added a new representative to the staff. H e is not particularly bright, he seldom gets the complete story, but nevertheless he is able to make hundreds of calls a month, the majority of which could not be made otherwise. And perhaps best of all, he works for a very modest salary. RECEIVED for review September 15, 1952. ACCEPTED March 8, 1953. Presented before the Division of Industrial and Engineering Chemistry, Symposium on Sampling as It Applies to Marketing of Chemioals, a t the CHEMICAL SOCIETY, Atlantic City, N . J. 122nd Meeting of the AMERICAN
Compounding of Silicone Rubber W. B. SPENCER, JR., W. B. DAVIS, AND F. L. KILBOURNE, JR. The Connecticut Hard Rubber Co., New Haven, Conn.
J. C.. MONTERMOSO Ofice of the Quartermaster General, Washington, D . C.
T
HE early silicone polymers were unimpressive in comparison
b
with other synthetic rubbers in appearance, rubberiness, milling properties, and physical properties. Had it not been for the useful properties under extreme temperature service conditions accompanied by good oil resistance, good compression set, good ozone resistance, and relative freedom from oxidative degradation at temperatures up t o 500” F., the silicones would not have progressed as far as they have. Detailed compounding information on the silicones has not been generally available up t o this time except in the patent literature. This situation is changing, and uncompounded polymer is available for those rubber manufacturers wishing to compound their own recipes. One of the most authoritative reviews of the chemistry and technology of the silicones has been brought up to date in a second edition by Rochow (7). Warrick (8) reveals considerable information concerning the compounding of polysiloxane elastomers with fillers. H e states that for strength, toughness, and high stretch, zinc oxide, titanium dioxide, and hydrated aluminum oxide are the best fillers. For small weight loss and slight dimensional change after long exposure, asbestos is a n excellent filler. For fast setting, iron oxide is the best filler. The magnitudes of stretch and tensile were low at the time of this application (1944). Warrick (9) has patented a range of silicone rubber compounds containing 20 t o 50 parts per 100 of polymer of a silica filler having a pore volume of 4 cc. per gram and a heat of wetting of 0.3 to 1.0 calorie per cc.
of pore volume. The proper degree of porosity, the desired heat of wetting, and the limits of concentration of pigment in the polymer are all held essential for the production of a tough, leathery compounded product which, after curing with benzoyl peroxide or tert-butyl perbenzoate, becomes relatively strong, efficient, and unusually solvent-resistant, Compounds with properties sueh as the following were reported: tensile strength 1203 pounds per square inch, elongation 70001,, efficiency 842. Efficiency is defined as the product of tensile and elongation divided by 1000. A journal article which has escaped attention is by Moakes and Pyne (6),who report an investigation of methods of curing and rate of change of physical properties of six early Silastic compounds, but do not reveal the recipes for the compounds. The authors report that the silicone compounds had poor tensile properties, good heat and cold resistance, a high initial compression set, followed by a slow rate of increase of set, no exceptional resistance to acids and alkalies, rather low swelling resistance in petroleum ether and aviation gasoline, good resistance to exposure cracking, poor abrasion resistance, comparatively constant resilience over a wide range of temperature, and good electrical properties. The most sensitive property for assessing state of vulcanization was found t o be compression set. Moakes and Pyne provided data showing the effect of temperature on the compression set test applied for 24 hours under 30% compression. Under the sponsorship of the Office of the Quartermaster General, The Connecticut Hard Rubber Co. has contracted to
1298
INDUSTRIAL AND ENGINEERING CHEMISTRY
study methods of improving silicone rubber compounds. Improvement in several directions simultaneously would be desirable-for example, in improved tensile strength and elongation, improved oil and fuel resistance, further improvement in compression set, and reduction of curing time and cost. All such improvements should be made without sacrifice of outstanding properties now generally recognized as characteristic of the silicones, notably wide temperature stability and freedom from oxidative degradation. Doede, Duke, and Glime ( 1 ) reported the early results arising from the preliminary testing of over 65 different pigments in the only raw silicone polymer available to them in 1948. These results, though generally disappointing, showed that some pigments increase tensile strength of silicone from about 25 pounds per square inch for the unpigmented polymer to approximately 400 pounds per square inch, and raise the elongation from about 150% to as much as 300%. The polydimethylsiloxane rubber used by these workers was known as General Electric 9979-G (no\\- known as SE-79) and was slightly cross linked, probably through the inclusion of a minor percentage of the trifunctional methyl trichlorosilane monomer. It contained iron salts, indicating that it probably was polymerized with ferric chloride as a catalyst. POLYMER AND PIGMENTS
Vol. 45, No. 6
structure of the pigment has not yet been revealed. The product information sheet put out by the Grasselli Chemicals Department, E. I. du Pont de Nemours &: Go., states that GS-199s silica has the following properties: Extremely small ultim'ate particle size of the order of 0.01 micron. High specific surface area, Pronounced hydrophobic and organophilic properties. Ease of dispersion in organic systems using conventional milling procedures. Surface area by nitrogen adsorption, 250 to 300 sa. meters Der gram. Bulk density, 7 pounds per cu. foot average. SiOn, 84 t o 88%. p H in 50-50 methanol-water mixture, 7.5 to 9.5. Physical form of silica, amorphous. It is chemically stable except in the presence of alkalies or hydrogen fluoride. Although it possesses much of the refractory nature of conventional silicas, it is subject to attack by free oxygen above 220' F. with loss of hydrophobic properties. It is expected t h a t GS-199s will ordinarily be used as a dispersion in a n organic medium, such as oil, and under these conditions the attack by oxygen is greatly reduced and in fact is often negligible. GS-199s silica has a strong reinforcing effect in natural rubber and in standard GR-S, cold GR-S, and Neoprene Type GN. Electron microscope pictures taken by this laboratory reveal that the tiny 0.01-micron particles are present in the form of aggregates ranging from 0.5 t o 2.0 microns in diameter. In this respect the GS-199s silica is similar to Santocel C. Titanox RANC (Titanium Division, National Lead Co.) is a rutile titanium dioxide whose ultimate particle diameter is of the order of 0.3 to 0.4 micron. It contains a minimum of 94% TiO, plus alumina, silica, and zinc oxide. It forms aggregates easily and probably is aggregated in the silicone rubber in view of the high loadings t h a t are possible. Alon is a very pure form of aluminum oxide produced experimentally by the Godfrey L. Cabot Co. Ultimate particle diameters lie between 0.02 and 0.04 micron with aggregates about 0.2 to 0.4 micron. The surface area is reported to be 50 to 70 square meters per gram. AIon is largely of the gamma crystalline form. It contains traces of iron and chlorides. The properties of Santocel C, D u Pont GS-l99S, Alon, and Titanox RANC are summarized in Table I.
I n I950 a new raw polymer, General Electric 81176 (now known as SE-76), appeared. This dimethylsiloxane polymer is a waterwhite, puttylike product which flows sufficiently to fill a container in which it is placed but is solid enough to band easily on a rubber mill. It is slowly volatile until compounded and vulcanized, iosing 8.4% upon heating in a shallow dish for 3 hours at 300' F. A t 480' F. for 10 hours, a sample lost 11.3% in weight. One outstanding advantage of the new polymer over the earlier one is the ease of incorporating reinforcing fillers. This is especially noticeable with extremely finely divided or highly surface active pigments. Either dibenzoyl peroxide or tert-butyl perbenzoate is a recommended curing agent. The rubber is reported to be rapid curing ( 4 ) ; a study of the rate of cure is given in detail herein. Recommended pigments for this rubber do not differ from those heretofore considered ( 1 ). Such pigments as titanium dioxide, zinc oxide, lithopone, fine-particle calcium carbonate, diatomaceous earths, ground silica gels, silica aerogels, alumina, etc., may all be TABLE I. PROPERTIES OF PIGMENTS employed, Of these, the authors have selected a silica aerogel (Santocel C), an Titanox Item R.4XC Santocel C GS-1998 Alon alumina (Alon), a t i t a n i u m d i o x i d e 94 ............ (Titanox RANC), and a new hydrophobic Present 5 ............ ....... 4.0-6 .O ............ silica (Du Pont GS-199s) to illustrate the T:2it$ organic, % ....... 2.5-3.5 ........... $9. . . . . . . . . . degree of reinforcement obtainable a t variA ~ ~% o ~ , Present ........... ........... Present ........... .... ............ ous volume loadings and under various ....... ........... Present ........... 6.5-7.5 3.5-4.0 7.5-9.5 5.4 conditions of surface pH, moisture, and ~ ~ g ~ microns g ~ ; ~ ~. . . .e . . . 3-5 0.5-2 0,2-0.4 0.006-0.014 0.02-0.04 Ultimate partible size micron 0.35 0.005-0.015 concentration of curing agent. 300 50-70 Santocel c is a silica aerogel containing ~ ~ ~ 260. . ' ' .~ 500 6 ~ 7~ ~ 4-5 ~ 4.2 2.2 1.98 3.6 approximately 89.5 t o 91.5% silicon diAmorphous Amorphous Largely gamma Specific Crystalline gravity state Rutile crystalline oxide, 2.5 to 3.5% sodium sulfate, 1% iron form and aluminum oxides, and 4 to 6% volatile matter according to the manufacturers (Monsanto Chemical (30.). The The dibenzoyl peroxide used in this work was Lucidol benzoyl p H (water suspension) is 3.5 to 4.0. Aggregate size is 3 t o 5 peroxide purified, obtainable from the Lucidol Division, Novamicrons, which is relatively coame, but the surface area is 500 del-Agene Corp. square meters per gram, which is high and indicates a high degree Except for removal of moisture, no special attempt to purify of porosity, The ultimate particles appear to average 0.01 micron these pigments was made. It was noted that the Alon sample from electron microscope examination. This fact and the ability contained somewhat more iron oxide as a n impurity than the of the pigment t o adsorb and hold moisture and t,o be wet by the specification indicates. Samples with different amounts of iron silicone rubber itself are significant in explaining the excellent reoxide. caused little variation in the properties imparted t o the inforcing properties of Santocel C. rubber; hence, this is not considered important. Results obLate in the year 1951, another finely divided pigment known tained with Alon were unusual, and for this reason it is felt that aommercially as D u Pont GS-199s hydrophobic silica became they should be reported t o illustrate the difference from the available. It had already been extensively evaluated as an inorsilica-type pigments. ganic thickening agent for oils to make lubricating greases. The
;::!g ,g g %: ,
~
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1953
1299
MIXING, CURING, AND TESTING
Mixing was carried out on a Thropp 3 X 8 inch laboratory mill equipped with a scraper blade. The rolls revolve at speeds of 44 and 36 r.p.m. The procedure was t o break down the rubber, which required only a minute or two, and then t o add pigment gradually. This required less than 30 minutes in all cases. Press curing was carried out with a n electrically heated Elmes 8 X 8 inch laboratory press, The press cure was for 15 minutes at 230' F. (except as otherwise noted) a t 550 pounds per square inch pressure, and the tern erature was measured with a thermometer inserted in a Eole drilled in the 6 X -6 inch slab mold. Oven curing was carried out in a Wheelco Capacitrol controlled electrically heated circulating air oven. Tensile and hardness tests were carried out in accordance with ASTM specifications D 42-141 and D 676-47T. Compression set tests were carried out in accordance with ASTM specification D 395-49TJ Method B.
ELON GAT ION
;c
@ 300. I
I
700. 'ss 600
I
m
I
N
W X)O~MODU
Is5OO.
RESULTS AND DISCUSSION
A.
PRESSAND OVENCURING. The curing reaction of benzoyl peroxide with polysiloxanes is one in which chemical bonds are created between molecules of polysiloxane. According t o Rochow ( 7 ) , the reaction appears to cause oxidation of methyl groups and subsequent cross linking between methylene groups. It is possible to determine the number of such cross links by swelling measurements and the use of theories of network structure of Flory and Rehner ( 3 ) . Such measurements have been made in this laboratory. Since the established technique of curing silicones with benzoyl peroxide depends on heating the mixture of the two ingredients at temperatures in excess of 100" C. (the approximate temperature of benzoyl peroxide decomposition), it would appear logical that the primary chemical bonds causing
5 50 W
S 40 30
700
,.z 600
b 900
'
400
300
1 900
'
3 TENSILE
800 700
PRBS CURE Figure 1. Physical Properties us. Press Cure GE 81223 Silicone Rubber Compound Oven Cure. 4&hour cycle to 480' F. Press Cures A. 5-minute rise to 210' F. C. 10-minute rise t o 230° F. 5 minutes at 230° F. E. 10-minute rise t o 230' F. 15 minutes a t 230' F. 6. 18-minute rise to 25Q0F. 16 minutes at 250° F. I . 15-minute rise to 300' F. 5 minutea at 300' F.
+++ +
60,
1
A
B
I e D OVEN CURE
HARDNESS I I E F
Figure 2. Physical Properties vs. Oven Cure GE 81223 Silicone Rubber Compound Press cure. 15 minutes a t 230' F.
Oven Cures F. B. 8 houirs at 300' F.
A. .-
1 hew
at 300°
C. 8 hours a t 300° F. f 16 hours at 400' F. D. 8 hours at 300' F. 16 hours a t 450' F. 2 hours a t 350' F. E. 8 hours a t 300' F. 16 hours a t 400' F. 8 hours a t 480' F. F. Like E 5 hours at 400-500° F. 2 hours at 500' F.
+ ++ + ++
vulcanization are a t least initiated and probably completely formed during the press curing. Subsequent heating, assuming the above, serves to remove by-products such as benzoic acid and water, resulting from the cross-linking reaction or from condensation of other unreacted (OH) groups in the molecule or released from the surface of pigments, and unreacted polysiloxane molecules or molecular fragments split off by thermal, oxidative, or hydrolytic actions. Because the reactivity of benzoyl peroxide a t press curing temperatures is great, it seemed advisable t o study the effect of variations in time and temperature of press curing. A complete compound, General Electric 81223 (now known as SR-450), was selected for this study. It is made from SE-76 dimethyl silicone polymer containing 45 parts of Santocel C and 1.65 parts of benzoyl peroxide per 100 of polymer. The periods and temperatures of curing were varied as shown in Figure 1. All slabs were placed in a cool mold, cured with the specified rige time and curing time, and removed after cooling the mold. All slabs were then cured in a circulating hot air oven using a 48hour, gradually rising temperature cycle ending at 480" F. Thus, the only difference between test slabs was the extent of the reaction in the press between benzoyl peroxide and the rubber. The data show that maximum stiffness and tensile were reached with a press cure of 15 minutes at 230' F. Maximum hardness and compression set were attained with a 10-minute cure at 260' F. Cures a t times and temperatures greater than these had the effect of reducing elongation and tensile. Shorter cures a t lower temperatures resulted in softer, less stiff vulcanizates with lower compression set. These data indicate that compression set of this recipe is greater the larger the proportion of cross links present. For this partkular recipe, a maximum press cure of 15 minutes a t 230' F., followed by an oven cure extending over 48 hours with maximum temperature of 480' F., is recommended. The effect of varying the oven cure was next investi-
gated. The 15-minute cure at 230" F. was selected as a standard press cure and a series of oven cures was run using the same recipe. These data, reported in Figure 2, show that maximum hardness and tensile are achieved with an 8-hour cure a t 300" F., and maximum stiffness is achieved with an oven cure of 8 hours a t 300' F. plus 16 hours a t 400' F. All cures a t temperatures over 300' F. resulted in reduced compression set, the result in cure E being attributed to experimental error. This was a relatively simple compound and was not especially designed for Ion. compression set. Low compression set in silicones may be imparted by the addition of oxides of mercury ( 5 ) . It was concluded from these two experiments that the primary curing reaction with benzoyl peroxide should be carried out under conditions which nil1 produce a maximum number of cross links but without overcuring in the press. A curing period of 15 minutes a t 230" F. n-ith a concentration of 1.65% of benzoyl peroxide appears to do this. The secondary or oven cure should be adjusted depending on the degree of heat resistance required. Maximum stiffness and tensile appear to be developed in 8 bo 16 hours at 300' to 400' F. Further improvement in compression set resistance will result from curing a t temperatures up to 500" F. It is obvious that the higher the expected service temperature, the higher the oven curing temperature should be. The fact that only slight changes result from prolonged high temperature curing of well-formulated silicone compounds is evidence that the oven curing is simply a LLclean-up"period necessarjto attain the highest degree of stability against further change during high temperature service, and is made a t a sacrifice of tensile and elongation and with a slight improvement in compression set resistance. A corresponding improvement in solvent resistance would be expected and was indeed found by Moakes and Pyne (6). SILICAAEROGELPIGMEST (SAKTOCEL C). The effect of increasing volume loadings of four pigments in General Electric SE-76 gum is reported herein. The first of these to be considered is Santocel C, a very porous silica aerogel sold by the Moneanto Chemical Co. The effect of increasing volume loadings of the pigment in the normal "as received" condition is shown in Table 11. Most silica-type pigments give strong reinforcement-that is, high modulus and low elongation stress-strain curves. Santocel C is exceptional in that, because of its large aggregate size, a fairly large volume of it can be milled into the rubber without ex-. cessive dryness of the batch. The batches tend to dry out on standing and even become boardy, possibly because of progressive adsorption of silicone rubber in the pores of the silica aggregates. Plasticity is restored by remilling. During cure, the wetting or adsorption of the rubber on the Santocel C is completed and the pigment-rubber geometrical relationships undoubtedly become fixed. The result is a stiff, moderately high tensile compound with not too great a hardness and moderate elongation. Maximum tensile strength occurs a t a volume loading of 15 to 20%.
C IX GENERAL ELECTRIC SE-76"tb TABLE 11. SANTOCEL Stress a t
Vol. Pigment/ 100 Rubber 5 10 15 20 25 27.5 30
Vol. 45, No. 6
INDUSTRIAL AND ENGINEERING CHEMISTRY
1300
ZOOYO, Lb./Sq. Inch 101 212 390 481 533 582
Tensile Strength, Lb./Sq. Inch 168 586 862 887 576 694
Elongation,
Hardness, Shore A
300 333 342 308
27 40 50 61 71 73
%
225
263
Slab cracked on molding a Curing agent 2 0% benzoyl peroxide. b Press cure 15 minutes at 230° F.; oven cure 1 hour a t 300' F.
TITANIUM DIOXIDE P I G X E K T ( TITANOX RBNC). Titanox RASC is reported in Table I11 and may be contrasted with Santocel C. Titanox RANG can be milled into the rubber in a t least triple the volume concentration possible v, ith Santocel C. RIoderately high tensiles are possible with Titanox RAKC without
too great a reduction in elongation. Here, as in most silicone compounds, elongations increase as pigment is added, reaching a maximum and then decreasing as the loading limit is approached, Whether the effect of the pigment is to prevent the occurrence or spreading of minute tears in the rubber structure ( 2 ) or whether the pigment simply has a greater attraction for the siloxane molecules than they have for themselves is not clear. Titanox RANC may be considered typical of the metal oxide type of pigment in silicone rubber. Optimum tensiles occur m-ith 40 to 50 volumes of Titanox RANC per 100 of rubber.
TABLE 111. TITANOX RAKC Val. Pigment/ 100 Rubber
IN
GENERAL ELECTRIC SE-76avb
Tensile Strength, Lb./Sq. Inch
Stress a t ZOO%, Lb./Sq. Inch
Elongation,
Hardnese, Shore A
%
34 38 46 52 56 55 71
Curingagent 2.0% benzoyl peroxide. F.; oven cure I hour a t 300' F.
b Press cure 15 minutes a t 230'
~
~
_
_
_
COATEDSILICAPIGMENT (Du PONTGS-199s). When GS199s silica was tested a t low loadings, the results showed nothing unusual except that high elongations were obtained for a silicatype pigment. At higher loadings, astonishingly high tensile strengths were found. Tests have shown that it is possible t o incorporate up t o 52.5 volumes of GS199d hydrophobic silica in 100 volumes of gum. With 52.5 volumes of silica, it appears that the pigment has used up all of the available rubber on its surface and there is none left over t o allow a slab of the compound to mold during the cure. These large loadings, as compared vr-ith Santocel C, are evidently possible because of a protective coating on the pigment. TABLEIV. GS-lgSS
IN
GENERAL ELECTRIC SE-76a16
Stress a t
Stress a t
Vol. Pigment/ 100 Rubber
Lb./Sq. Inch
Lb./Sq. Inch
10
85 242 372 410 485
197 542 82 0 79 5 925
15 20 25 30 Q
b
20070,
400%,
Tensile Strength, Lb./Sq. Elan ation, Inch 226 1170 1900 1670 1365
k
Hardness, Shore A
475 613 625 625 525
23 61 68 80 80
Curing agent 2.0% benzoyl peroxide. Press cure 15 minutes at 230' F.; oven cure 1 hour a t 300' F.
I n Tahle IV results are given shoning the high elongation and tensiles possible with GS-199s silica. The values obtained for tensile strength and elongation in Table IV are obviously outstanding as compared n-ith any heretofore found and reported with any other pigment nhen used in silicone rubber. GS-199s silica produces harder stocks with lower moduli than Santocel C. It is concluded that the hydrophobic pigment aggregates are thoroughly dispersed in the silicone rubber, but that the ultimate particle surfaces of the silica are probably not IT-et by the rubber, as in the case of the Santocel C (Table 11). Table I T T gives representative results obtained with 10 to 30 volumes of GS-199s silica with a 1-hour cure at 300" F. Data shown in Table V illustrate the effect of longer curing a t 300" F. as well as a t higher temperatures and Tyith reduced percentages of benzoyl peroxide. It will be seen that the longer, higher temperature cures result in increased hardnesses, lower elongations, and higher moduli. Some cures a t 400" F. resulted in elongations less than 100%. These compounds r e r e excessively stiff and hard. In other cases, the compounds withstood curing for 24 hours a t 400" F. without becoming hard. This table illustrates also the advantage of high temperature curing to reduce compression set.
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1953
1301
known. It is obvious that two types of curing action take place when benzoyl peroxide is present. At press curing Stress Tensile Benzoyl a t ZOO%, Strength, ElongaHardtemperatures (200' t o 300" F.), the curVal. Pigment/ Peroxide, Oven Cure Lb./Sq. Lb./Sq. tion, ness Compression 100 Rubber Yo Hours F. Inch Inch % Shore'A Seta, % ing action of GS-1998 is small. It be1008 525 61 .. 300 346 2 1 15 comes increasingly active as the curing 2 24 300 597 975 350 68 83 temperature is increased. Under most 2 24 400 795 795 200 76 62 conditions, it has been impossible to 2 1 300 419 1615 613 77 95 25 2 24 300 684 1170 388 85 85 cure a t 480' F. without causing extensive 2 24 400 ... 795 75 93 49 stiffening of the rubber, which then has 1 1 300 242 1170 613 61 .. 15 1 24 300 274 840 475 63 86 properties intermediate between a rubber 1 24 400 630 710 275 70 45 and a resin. 1 1 300 276 1930 850 73 97 25 The results presented here are suffi1 24 300 554 1660 550 83 87 1 24 400 .. . 795 63 93 42 ciently outstanding to excite the imagina15 0 1 300 95 855 1025 36 .. tion of compounders who have been ac24 300 210 1660 913 53 79 24 400 387 1160 638 63 40 customed to silicone rubber compounds 0 1 300 165 1245 1000 62 97 25 having only 200 to 500 pounds' tensile 24 300 298 1720 775 70 91 strength and 50 t o 200% elongation. 24 400 782 897 275 89 65 a Press cure 15 minutes a t 230° F.; oven cure as indicated. The often-experienced hope or convicb Compressionset, per cent original deflection after 70 hours a t 300° F. by ASTM D 395-49T (B). tion that silicone rubber compounding would eventually come of age with compounds showing physical properties * comparable to those of some of the other special purpose rubbers is a distinct probability. ALUMINUM OXIDEPIGMENT (ALoN). The fourth pigment to be discussed, Alon, gives very unusual results. I t s high degree of 600 subdivision and behavior in General Electric 9979-G indicated t h a t it would give a high degree of reinforcement to the softer, more plastic polymer, SE-76, and this was found to be true. As larger and larger volume loadings of Alon were added, more and more benzoyl peroxide was required. It was possible to prevent cure entirely by raising the volume loading of Alon above I I I I I 15 volumes when using only 2% of benzoyl peroxide. It appears that Alon inactivates benzoyl peroxide or catalyzes its decomposition in such a way as to inhibit the formation of cross links. When this occurs, limited cure takes place, and very a 0 stretchy, snappy test pieces result with as much as 1400% elon20 gation and 1000 pounds per square inch tensile but with low hardness values. Such compounds are unstable with respect t o further curing and may revert if too few cross links are present. When additional curative is used, increased stability is attained. 300 r' Tables VI and VI1 present a fairly complete survey of the possibilities of Alon. Table VI1 shows how the curative-robbing too characteristics of Alon may be offset by adding extra curing agent. 4 5 PH 6 7
TABLE V.
GS-199s
IN
GENERAL ELECTRIC SE-76"
m
2
Figure 3. Physical Properties US. pH of Pigment Alon in SE-76 Silioone Gum 10 volumes, 4.0% benzoyl peroxide
TABLE VI. ALONIN SE-76 SILICONER U B B E X ~ ~ + Tensile Strength Elongation, Lb./Sq. Idch % 0 29 125 2.5 58 208 5.0 140 260 10.0 585 795 12.5 537 845 15.0 826 1350 811 1250 15.0 4 300 15.0 736 1065 Curing agent 2.0% benzoyl peroxide. b Press cure 1.5 minutes a t 230' F.; oven cure as indicated.
Vol. Alon/ 100 Rubber
The effect of reduction of the benzoyl peroxide concentration from 2 to 0% is t o reduce the rate of cure, increase elongations, and decrease hardness. There was a reduction in the excessive stiffening effect noted above when lower concentrations of curing agent were used. There appears to be an advantage in using the lower concentrations of benzoyl peroxide with the 15 volume pigment loading from the standpoint of compression set. The 1930-pound tensile strength figure is the best that the authors have ever recorded on silicone rubber. With no added benzoyl peroxide, the rate of cure is reduced so that optimum tensile is reached, not in 1 to 8 hours at 300' F., as is usually the case, but with a more intensive cure. Other data not shown here indicate the optimum cure a t 400' F. to be about 16 hours in the absence of benzoyl peroxide. As far as is known, TableV is the first publication of data demonstrating t h a t silicone rubber can be given a high degree of cure in the absence of added peroxides or other oxidizing agents. The exact mechanism by which this cure is obtained is not yet
Oven cure Hours F. 1 300
Hardness, Shore A 26 35 35 37 33 32 28 27
TABLE VII. 20 VOLUXESOF ALONWITH VARYINUBENZOYL PEROXIDEQ
Stress a t Tensile Benzoyl 400% Strength Elongation, Hardness, Peroxide, % Lb./Sq. inch Lb./Sq. Inbh % Shore A 139 2.0 245 25 750 260 783 3.0 603 35 383 3.5 748 38 667 42 468 617 4.0 818 4.5 812 42 540 542 932 556 5.0 40 583 a Press cure 15 minutes at 230' F.; oven cure 16 hours a t 400' F.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1302
Comparison of Tables VI and VI1 4 i t h Table I1 shows the marked contrast between silica and Alon. Santocel C achieves moderately high tensile by strongly reinforcing the rubber, as evidenced by the moderately stiff and hard vulcanizates. Alon compounds, on the other hand, stretch easily and achieve high elongation and tensile with softer products. The softer Alon vulcanizates may be presumed to be cross linked to a lesser degree, as it is obvious that Alon interferes somewhat with the
s m
-I
5
tendency toward even higher elongation than those already noted as unusually high for the normal pigment. Figure 4 shows the data obtained with Santocel C a t 16.6 volume loading and 4% benzoyl peroxide. Here optimum reinforcement is in the pH range 4 to 6. As a variance from Alon behavior, it is noted that Santocel C gives lower elongations on either side of this range. High elongation in silicone compounds is obtained with zinc oxide (pH 7 to 8), calcium carbonate (pH 10 to ll),and hydrated aluminum oxides (pH 9.4). The silicas generally result in low elongation compounds with the possible exception of hydrophobic silica GS-199s. The effect of pH on reinforcement is probably related to the effect of the individual pigment on the decomposition of benzoyl peroxide. Furthermore, a highly alkaline or
700
,. 600
300
200
z9
5 400 W
70
8
5
s
50
k! 200 a 70
2 700
s3 500
2 Figure 4.
Vol. 45, No. 6
W
w 0 I
v)
30
4 pH 6 Physical Properties us. pH of Pigment Santocel C in SE-76 Silicone Gum 16.6 volumes, 4.0% benzoyl peroxide
vulcanization reaction. This is shown in Table VI, where increasing volume loadings of Alon fail to result in harder vulcanizates but actually in softer ones. INFLUENCE OF pH OF PI~MENT. I n order t o investigate further the nature of the pigment-silicone bonds, experiments were carried out in which the surface pH of pigments was changed. In this work, the pigment was exposed to dry hydrogen chloride or ammonia at room temperature and pressure, Following this treatment, excess reagent was removed by heating and evacuating for 3 hours. The pH of the pigment was measured by suspending 0.5 gram in 25 ml. of boiled distilled water and measuring the pH of the suspension with a Beckman pH meter. The treated pigments were then compounded a t moderate volume loadings and with a percentage of benzoyl peroxide known to be sufficient. Figure 3 shows the data obtained with Alon. The maximum reinforcement was obtained with samples having a pH of approximately 5.5. On either side of this point, Alon showed a
PERCENT WT LOSS
..
Figure 5. Physical Properties us. Weight Loss of Pigment on Heating at 200' to 1700" F. Santocel C in SE-76 silicone g u m , 15 volumes, 2.0% benzol peroxide
acidic pigment rnay cause depolymerization of the silicones. Such side reactions caused by the pigment surface appear to go on without causing cross linking of the silicone molecules. It is evident from the work done here with Santocel C and Alan that both types of pigment impart greatest reinforcement to SE-76 rubber when in the neutral (neither strongly acid nor alkaline) state. INFLUENCE OF WATERCOXTBXT OF PIQMENT.Santocel C and Alon both tend to adsorb moisture from the air. These pigments mere thoroughly wetted bg exposure to saturated water vapor for 24 hours. They were then heated a t increasing. temDerature levels to drive off adsorbed moisture and were compounded in SE-76 rubber. The loss of TABLEVIII. SANTOCEL C WITH VARYIKGWATER CONTENT^ weight (moisture) and pH of the rePigment Physical Propertiesb sultant pigment and physical tests obTime and Temp. of Stress at Tensile Heating Pigment Pigment 200% strength, Elan ation, Hardness, tained are shown in Tables VI11 and I X F. Wt. Loss, % ' p H (HzO) lb./sq. i h h lb./sq. inch Shore h Hours and are plotted in Figures 5 and 6. T o t heated 3.88 333 576 338 53 6 220 5'.40 3.86 367 577 325 62 When heated a t temperatures to 1000' F. 4 500 4.82 3.85 421 559 276 60 Santocel C loses up to 6% moisture 3 1000 6.38 4.23 506 506 200 66 3 1500 8.00 8.20 202 234 250 45 with an appreciable improvement in 1 1700 7.35 8.17 ... 101 175 30 reinforcement as measured by modulus 0 Compounded with GE SE-76 silicone gum, 15 volumes per 100 volumes of gum, 2.0% benzoyl or hardness. At 1500' F. or higher temperoxide. b Press cure 15 minutes a t 250' F.: oven cure 1 hour a t 300' F. peratures, the pigment sinters and pH of 0.5 gram of pigment in 25 ml. of boiled distilled water (pH 6.8 to 7.0). properties fall off. Y
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INDUSTRIAL AND ENGINEERING CHEMISTRY
30
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the newer rubber, SE-76. The pigments are listed in order of decreasing effectiveness, as measured arbitrarily ~y the product of tensileand elongation. This is the "efficiency" index defined by Warrick (9). At this time Alon, an aluminum oxide, and GS-199s hydrophobic silica head the list, followed not too closely bv several silicas and metal oxides. It is the authors' belief that the fine silicas and aluminas offer the best reinforcement for silicone rubber but that the oxides of zinc and titanium also have useful properties. Where tensile properties are less important than cost and exteasibility, the
of silicone products through reinforcement are now possible. The effect of moisture and surface pH of Santocel C and Alon has been studied. Neutral, dry surfaces are desirable for maximum reinforcement. I n vulcanizing with benzoyl peroxide, significant differences in properties result if the press cure is not carefully specified. A
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Suggestions made by Warren Stubblebine and C. C. Vogt of the Office of the Quartermaster General and the assistance of other individuals of the laboratories of The Connecticut Hard Rubber Co. are much appreciated. The inspiration provided by C. M. Doede of the latter company is especially appreciated. LITERATURE CITED
(1) Doede, C. M., Duke, N. A., and Glime, A. C., paper presented before Division of Rubber Chemistry, AM. CHEM.Soc., Wash-
inaton. D. C.. March 1951. (2) Fairbank, H. A:, Walker, C. A,, and Doede, C. M., Phus. Rev., 83, 205 (1951).
(3) Flory, P. J., and Rehner, J., J . Chem. Phys., 11,512 (1943).
Vol. 45, No. 6
(4) General Electric Co., Technical report on 81176 silicone polymer, April 3, 1951. (5) Jones. H. F., U. S. Patent 2,448,530 (Sept. 7, 1948). ( 6 ) Moakes, R. C. W., and Pyne, J. R., J . Rubber Research, 19, 77 (1950). ( 7 ) Rochow, E. G., “Chemistry of the Silicones,” 2nd ed., Kew York, John Wiley & Sons, 1951. (8) Warrick, E. L., U. S. Patent2,460,795 (Feb. 1, 1949). (9) Ibid., 2,541,137 (Feb. 13,1951). RECEIVEDfor review November 4. 1952. ACCEPTEDFebruary 20, 1953. Presented before t,he Division of Rubber Chemistry, ANERICANCHEMICAL SOCIETY,Buffalo, N. Y., 1952. Work done under G. S. Government Contrao t D-4-44- 109-QM-64, 195 1.
Polymerization of G eratures J
REVIEW OF RECENT DEVELOPMENTS L. H. HOWLAND, V. C. NEKLUTIN, R. L. PROVOST, AND F. A. MAUGER hTaugatuckChemical Division, United States Rubber Co., Naugatuck, Conn.
T
HE period immediately subsequent t o widespread acceptance of “cold” rubber as a superior tire tread polymer was marked by extremely rapid development of formulations for the polymerization of the new synthetic rubber. Efforts in this direction were sparked by the necessity for the GR-S producers to arrive at recipes which would permit maximum flexibility of operation, uniformity, economy, and ease in adjustment of reaction rates, and a t the same time eliminate certain objections to the early formulations such as tendencies toward latex instability. Neklutin et at. (9) gave some indication of the wide variety of formulations applicable to polymerization of GR-S at 41’ F. and mentioned the commercial status of several recipes. The present paper reviews more recent developments by the authors in polymerization techniques and mentions several developments now in the pilot plant stage which show promise of emergence to commercial status in the near future. POLYMERIZATION RECIPES
41° F. (STANDARD COLDRUBBER). The original sugar-free, ferrous pyrophosphate recipe described by Neklutin et al. (9) has been changed very little (Recipe I, Table I). T h e substitution of diisopropylbenaene monohydroperoxide (or p-menthane hydroperoxide) for the original cumene hydroperoxide has permitted a reduction in the ferrous sulfate-potassium pyrophosphate levels required to attain 60% conversion of monomers in the same length of time (approximately 14 hours). This recipe has been used commercially t o make GR-S 101 in the government-operated synthetic rubber plants. The sensitivity of the recipe to activator make-up, a t first considered disadvantageous, has since been overcome by improvements in preparation techniques, Just as the sugar-free, ferrous pyrophosphate recipe has been preferred in polymerizations where a t least part of the emulsifier is rosin soap, polymerizations emulsified with all-fatty-acid soap a t the present time employ a so-called polyamine activation system in place of ferrous pyrophosphate. The polyamine activators, first described by Whitby e t al. (17) and later studied extensively by other investigators (5,9),have been found to be very versatile, and the lack of necessity for complex activator
preparations has resulted in excellent reproducibility of reaction rates. I n polymerizations employing all-fatty-acid soap emulsification a t 41” F. (Recipe 11, Table I), diethylenetriamine (DETA) has been used in preference t o the amines of higher molecular weight such as the more active triethylenetetramine or tetraethylenepentamine, because of its economy, uniformity, and availability. Diethylenetriamine was the activator employed in the commercial production of GR-S X-565 mentioned in a previous paper (9) and i t has since been used in almost all cold GR-S where the polymerization emulsifier has been 100% fatty acid soap. Diethylenetriamine activation has also been employed in many cold, high-solids foam sponge latices, some of which are emulsified with mixtures of fatty acid and rosin acid soaps. One recent development which has improved the versatility of the diethylenetriamine-activated recipe is the use of small amounts of ferrous sulfate in conjunction with the polyamine. The effect of these small amounts of ferrous sulfate on reaction rate is shown in Figure 1. The iron salt is dissolved in the diethylenetriamine solution, so that charging is not complicated in any way. Sometimes it is desirable to use a small amount of sequestering agent along with the added iron, A recipe for polymerization of a rosin soap (Dresinate 731)emulsified system with tetraethylenepentamine has been re ported (9) and sequestering agents such as ethylenediaminetetraacetic acid have been found to have a beneficial effect in the recipe ( 1 1 ) . Several polymers have since been produced on a commercial scale using tetraethylenepentamine a8 the activator, but the nonuniformity and shortage of this amine have prevented extensive use of this system up to the present time. Development of 41” F. recipes employing rosin soap emulsification with the lower polyamines was retarded by the fact that they were considerably less active than the pentamine and reaction times were excessively long even a t high activator levels. The discovery that strong inorganic reducing agents have a beneficial effect on reaction rate when the lower polyamines are employed as activators (4)was thoroughly investigated. It was found that the most important variable in such a formulation was the amount of reducing agent added, the limiting conversion
