Capacitors Filled with Silicone Fluids
Structure and Properties K. R. MCGREGQR Mellon Institute, 4400 Fifth .Ize., P i t t s b u r g h , Pa.
T h e unique properties of silicones result from the inherent properties of the siloxane bond. The siloxane structure is stable to oxidation, although i t is susceptible to rearrangement at somewhat elevated temperatures. Its polarity results in an ability to orient on many types of surfaces such as metals, glass, and textiles. The freedom of rotation of the silicon-oxygen linkage allows linear molecules to assume a helical form; the gradrial opening of this helix with increasing temperatures is responsible for the good temperature-viscosity slopes of the liquids. Other properties may be superimposed by the choice of type and amount of organic groups attached to silicon. The selection of the average degree of functionality present in the monomers can bring about further alterations, as can the technique of polymerization. Through all these changes the siloxane bond maintains a sturdy framework.
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RIGINALLY those elitering the silicone field saw tlic evident properties of heat resistance and electrical insulntion. But even before this field could be properly cultivated, other values became evidtxnt-water repellency, inertnesfi to atmospheric attack, ability to inhibit foams, disregard for extremely low temperatures. A survey of some of the known properties of silicones r i t h a consideration of the reasons for their presence may suggest other properties that may be found or developed. H E A I RESISTANCE
I’crliaps the best recognized property of silicones is that of heat resistance. It may not be surprising to think that a polymer htiving the same type of structure found in quartz should he st:Ll)le a t high temperature, for that type of grouping is not susceptible to further oxidat’ion. But organic groups are necessary components of all silicones, arid what is to prevent them from being destroyed by heat a t the same temperature as other organic mnterials? Whatever may be the reason, the fact remains that thr organic part of a silicone maintains its integrity a t temperat u r w considerably above those a t which fully organic bodies re-
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main stable. One can do little more than make an intelligent guess as to why this should be, and one statement that sounds reasonable is to the effect that The silicon-oxygen bonds of about 50% ionic character may be thought of as powerful internal dipoles which act to decrease the external fields of substituent groups, and the general result is a decreased influence of perturbing agents such as light, or a reduced effect of force fields of other molecules tending to react with the hydrocarbon portion of the siloxane ( 8 ) . Such an explanation would fit in with the observed rapid loss of heat stability as the alkyl groups on silicon are lengthened from methyl to ethyl and beyond. The farther the carbons in the chain are removed from silicon the less the protective effect. .Alkyl carbons which are 3 or 4 removed from silicon are no more resistant to oxidatioii than if they mere parts of a purely organic compound. Phenyl groups on silicon are highly resistant to oxidation, presumably again because of the protective action of the siloxane dipoles, coupled with the fact that the aromatic compounds are, by their nature, more resistant to oxidation than the paraffins. Thus if heat stability is the only requirement of a silicone, the organic groups should be methyl or phenyl
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TEMPERATURE-VISCOSITY SLOPE
A second property which is well recognized is that of small viscosity change of the silicone fluids with change of temperature. Again, the reasons for this are found in the Rtructure of the base chain of the polymer. The large size of the silicon atom (compared with carbon), the large bond angle with oxygen, and the ionic character of the siloxane bond permit a great freedom of rotation or a very flexible molecule. At rest, the molecule takes the form of a coil which may be relatively tight or loose deprnding on the temperature applied. .4s the temperature is raised there is an uncoiling or relative straightening out of the molecule. The straightened molecule has a higher viscosity than it has in the coiled form, and this increase in viscosity due to change in shape balances, to some degree, the decrease in viscosity due to thermal effects (2). Further than that, the dipoles which were internally compensated in the fully coiled form may, as uncoiling takes place, show interaction between adjacent chains, still further combatting the loss of viscosity due to temperature rise Bulky substituents on the silicon atoms that would restrict the ability to coil bring about larger viscosity changes with tenipeiature. For instance, linear siloxanes in which both phenyl and methyl groups are present show a greater viscosity change than those that have all methyl groups. Even the methyl groups are not without some restricting effect, for the fluids of the least change that have been reported are those described by Wilcock and Hurd in which the methyl groups are largely replaced by hydrogen (+$) There may be other factors, for oxygens between the silicons may be replaced by atoms such as carbon. The flexihility of the carbon-silicon bond is less than that of the oxygen-silicon bond partly because of the difference in bond angle, partly because of the loss of the Si-0 dipole in favor of the low Si-C dipole, and partly because it has much smaller ionic character. Thus its viscosity response to change in temperature is more marked than that of the siloxanes, for there is less changc in the length of the chain with application of heat. Thus, for the best viscosity-temperature relations, the siloxane bond should be retained and the other groups on silicon should be small. WATER REPELLENCY
The third property of silicones that is outstanding is their m-ater repellency. Such wide use is made of this property that the impression seems to be abroad that the silicones are unique in this respect. i2s a matter of fact there is nothing unique about their water repellency per se. Organic waxes and similar hydrocarbons show about the same contact angle with water. The unique feature is not water repellency itself, but the ability to maintain this property when applied to a surface. The ability to stay put on a surface stems directly from the siloxane bonds. When spread on a glass or metal surface, residual hydroxyl groups of the silicone may condense with similar groups on the glass or metal, or hydrogen bonding may come into play between the hydroxyls on the surface of the solid and siloxane bonding3 in the silicone. This orients the siloxane part of the molecule to the base and exposes the organic and water-repellent part of the molecule to the moisture. Many types of textiles respond to this type of treatment as well, although it may be assisted by a certain amount of cross linking. The application of heat greatly improves the life of the films. The heat removes residual water from the base so that better contact is obtained. It promotes condensation between hydroxvls on silicon and base, and assists hydrogen bonding. Pcrhaps the most important contribution is that the heat causes an uncoiling of the silirone molecule so that more siloxane bonds can come in contact with the base. The net result is an anchoring of the silicone to the base on which it is placed, at the same time expoe-
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ing only the wat'er-repellent side chains. This anchoring of the film by siloxane bonding rather than simple wetting by the film is the reason for its long life. It also explains why heating improves the life. The properties of the siloxane bond and of silicon itself have been shown to be responsible for the ability of silicones to function in desirable ways, but there are other factors that should be noted that contribute to the great versatility of silicones. One of these is the fact that a silicone monomer may have a functionality of 0, 1,2, 3, or 4. Such a wide variation of functionality, while retaining the same basic structure, is not known in other types of polymers. Therefore, there may be silicon-containing compounds in the form of a gas, a volatile liquid, nonvolatile liquids of a great range of viscosities, thermoset resins, and rubbers. The proper selection of monomers may result in polymers in which the units vary in functionality from 1 to 4, with the average functionalitp anywhere between these two limits. Within this enormous range there is the possibility of preparing silicones in almost any physical form. The phrase "almost any physical form" is used advisedly, for there is one type of material which, so far, has not heen prepared; that type is a useful thermoplastic compound. The lack of this type of material results from the small change of viscosity with temperature. Polymers such as those from styrene and vinyl acetate soften enough a t elevated temperatures to allow them to be, formed. -4s they cool down the viscosity rises to the point where they have good rigidity and can serve many practical purposes As for the silicones, the change of viscosity is so small that, there is not enough difference between high and low temperature t o give a useful molded article. The absence of strong intermolecular force between molecules is another factor in the lack of stiffness in the thermoplastic silicones. For this reason, rigidity must be arrived at by cross linking monomers with funct'ionalities of 3 or 4. Here the knoxvledge o f silicone polymerization must be combined with the art of silicone polymerization. As far as the effect of cross linking is concerned, if t.here is too much, it mill become apparent in brittlmess; too little will bring about softness. The proper amount, of cross linking for a given purpose must generally be determined empirically. Of course, as cross linking develops insolubility, the time and the means by mhich it is to be brought about must be under control, and the knowledge of the structure desired must he supplemented by the art of preparing it. So far only the properties of silicon and the siloxane bond in affecting t'he properties of the finished silicone have been discusscd. One other very important factor to be taken into account is that. of the organic groups attached to silicon. The use of methyl groups in a silicone fluid is responsible for a smaller change of viscosity with temperature than t'he use of phenyl groups. However, the presence of phenyl groups has the advantage of bringing about a lower freezing point by interrupting the regularity of the polymer structure. Another effect takes place a t elevated temperature, for the phenyl groups are more resisbant to oxidation than are alkyl groups. Thus the polymers cont'aining both these organic substituents have a greater temperature span of usefulness, alt'hough this must be paid for in somewhat poorer viscositytJemperature relations. I n the field of resins properties such as toughness and handle-. ability are required, and monomers must be prepared with this in mind. The nat,ure of the resinous polymers prepared differs with the number and t8ypeof the organic groups on silicon, and selection must be made among the many available. The protective action of silicon fades out as the organic group increases in size. A compromise must often be made between the properties sought and those that are lost by dilution of the silicone part of the molecule. So far most of the commercial resins have only methyl and phenyl groups as organic substituents. This seems like a pitifully small selection from the wealth of those available, hut lha
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-Siliconesreasons for this are evident. Silicones are often selected for an application solely on the basis of their heat stabilily. , When methyl and phenyl groups are the organic substituents on silicon the best heat stability is obtained, and this stability results from proximity to the siloxane bond. Longer alkyl groups and larger aryl groups are more readily affected by heat, but many uses are being developed that demand little in the way of high temperature stability. The presence of silicon and the siloxane bond in larger or more complicated units may be expected to influence the properties sufficiently to make them desirable for applications that are totally different from any developed so far. The basic structure of silicones is responsible for properties that can be stated in a negative fashion. The usefulness of silicones is often not due to what they do, but to what they do not do. They do not oxidize readily; they do not hydrolyze readily; they do not show the conventional reactions to temperature changes; they are not good adhesives, they are not good solvents. This negative attitude has been turned to advantage in many applications such as mold release and electrical insulation. In the former case they do not dissolve in the molded article and do not adhere. I n the latter case they do not oxidize and they do not absorb water, two of the primary requirements of a good insulator. Within the last few years, the pharmaceutical and medical professions have been considering the possibilities of using this type of material, largely because it is inert, as a component of dermatological creams, and even as an agent for the treatment of lobar pneumonia. Silicones appear to be inert to metabolib processes and are such poor solvents for vitamins that there is little danger of vitamin depletion if they are ingested (1).
So the properties of a silicone product, be it fluid, resin, or rubber, stem from the properties of the siloxane bond-its large bond angle, its ionic character, and its dipole effects--the size of the silicon atom, the degree of functionality chosen, and the types of organic substituents on silicon. Each of these factors may be altered a t will. The siloxane bond may be substituted in whole or in part by bondings such as silcarbane or silazane; silicon itself may be exchanged for carbon; functionality may be from 0 to 4; the entire wealt,h of organic radicals is available for altering properties. This astonishing breadth of possible properties is the exciting thing about silicone chemiptry and is the reason for the confidence that further investigating of silicone structure will be rewarded by finding values greater than those yet seen. ACKNOWLEDGMENT
Acknowledgment is made to Dow Corning Corp. and Corning Glass Works who Rponqored the multiple fellowship a t the Mellon Institute, Pittsburgh, Pa., under n-hich this work was done. LITERATURE CITED
(1) Barondes, R. de R., Judge, W. D., Towne, C. G., Baxter, M. L., The Military Surgeon, 106, 381 (1950). (2) Fox, H. W., Taylor, P. W.,and Zisman, W. A,, IND.ENG CHEM., 39, 140’3 (1947). (3) Hunter, RI. J., Warrick, E. L., Hyde, J. F., and Currie, C. C. J . Am. Chem. Soc., 68, 2290 (1946). (4) Wilcock, D. F., and Hurd, D. T., U. S. Patent 2,547,678 (April 3, 1951). RECEIVED for review March 25, 1954
ACCEPTED June 4 , 19.54
ilieone Resins in Textiles FRED FORTESS Celanese Corporation of America, S u m m i t , N . J .
Apparel fabrics require finishing treatments to enhance their general performance and to overcome specific shortcomings for special end uses. The silicone resins based on liquid methylsiloxane polymers containing reactive groups for subsequent cross linking on the fabric have been found to impart durable water repellency, resistance to aqueous-borne stains, increased tear strength and abrasion resistance, and improved sewability, as well as recovery from wrinkling. The relationship of the resin structure to the properties imparted to the fabric is described. Optimum application conditions and sources of difficulties are outlined.
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7S GENERAL, the textile industry depends on a careful se-
lection of the fiber or fiber blends and on fabric construction to achieve optimum fabric performance for particular apparel end uses. Finishing treatments of the dyed fabrics with a variety of chemicals are frequently required to enhance important fabric properties and to minimize secondary fabric deficiencies. The application of silicone resins as fabric finishing treatments has been found to improve many functional and esthetic properties of a large variety of fabrics. A description of some of the useful properties imparted to textile materials has been recently discussed in the literature (1, 3, 6, 6 ) . As a fabric finish, the unique contribution of the silicones applied to the surface of fibers is their effectiveness a t lower levels of application and their durability, especially on the relatively unreactive hydrophobic fibers such as acetate, Dacron (Du Pont November 1954
polyester fiber), nylon, and the acrylics. The achievement of durable water repellency, improved abrasion resistance and tear strength, increased recovery from wrinkling, resistance to aqueous-borne stains, improved sen-ability, and the modification and improvement of the drape and hand or feel of the fabrics, has resulted from 10 years of intensive study of special silicone polymers and their adaptation to the complex processes of the textile dyeing and finishing industry. I n general these improvements in fabric properties can be related to the hydrophobic, tough, resilient film of silicone resin formed around each fiber. Many difficulties were experienced in developing a satisfactory silicone finish during the early investigations in commercial dyeing and finishing mills due to improper selections of resin structures, poor choice of catalyst and curing conditions, inadequate preparation of fabric and poor emulsification of the silicone. The current
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