Silicone Molding Resins H. N. HORIEYER, J. H. PRESTON, AND S. CASAPULLA The Connecticut Hard Rubber Co., New Haven, Conn.
E. R3. BEEKMAN Squier Signal Laboratory, Fort Monmouth,
N. J.
Recent aircraft and communications developments have resulted in operating conditions too severe for organic molded electrical insulating parts. An evaluation of silicone molding compounds for Army-Navy electrical connector inserts has shown that new type compounds have been developed that prevent insulation failure at very high humidity, at very high temperature, and from electrical arcing. Furthermore, their physical strength is equal to that o€ organic types. They can be readily molded with the short cycles and simple techniques used for phenolic compounds and long postmold cures, formerly required, have been eliminated.
E"""
since electricity has been commercially uscd, the pioblem Qf providing a completely satisfactory molded electrical insulating material has becn a recurrent one. A number of organic polymers have been used with considerable commercial success but continued use has always demonstrated basic limitations that have restricted the operating conditions of these materials. Recent developments in silicone molding resin technology have produced materials that overcome the basic shortcomings of the organic insulating materials, and at the same time, allow them to be used in standard equipment by feasible cominercial techniques. LI.MITATIONS OF ORGANIC COMPOUNDS IN ELECTRICAL INSULATIOS SERVICE
.
The strenuous operating requirements of modern aircraft have emphasized the limitations of the organic materials. Extremes of temperature and humidity are t,he rule rather than the exception. I n supersonic aircraft and guided missiles, temperatures far beyond the maximum allowable for organic materials are found. At high altitudes the extremely low temperatures for part's not directly exposed to the heat of the combustion process cause condensation of moisture from the air occluded within the various parts of the plane. This, in a great many cases, has caused operating failure of the electrical equipment involved. An ideal insulation material would have adequate mechanical properties, and Tyould mairitain high electrical insulation resistance a t all temperatures, both high and low, and a t the same time, would withstand high humidity. Continued use of the molded organic materials, however, has shown that each has certain inherent limitations. The principal drawback of all these molded materials lies in their unsatisfactory performance a t high temperatures and a t high humidity. The phenolic resins, which have been widely used since their commercial development by Ihelieland in 1908 ( 2 ) in addition to the t,emperature limitations, also are very deficient in arc resistance. Electrical discharge over the resin surface causes the formation of an electrically conductive carbon path. Melamine formaldehyde resins ( I S ) solved the carbon tracking problcm of the phenolics but introduced new difficulties. The molded pieces continued t o shrink a t room temperature for very long periods of time after removal from the molds. They also had inferior resistance t>ovibration. I n an effort to overcome November 1954
these limitations, neoprene rubber was used for some applications. It had excellent vibration-resistance, and could eliminatc most of the moisture difficulties by proper mechanical design. IIon-ever, it had inadequate resistance to extreme temperatures, inferior arc resistance, and hardened on aging by crystallization Furthermore, a serious problem rose in attempting to retain metallic inserts adequately. A recent material, diallyl phthalate, (9, 14), is a considerable improvement over thc earlier insulators, but also has some temperature limitations. Silicone resins were selected for the present investigation because they possess excellent stability to high temperature in addition to excellent electrical properties. Their inherent moistui e resistance also promised to overcome dielectric failure attributable to condensation of water on the insulation surface. The results indicated that silicone molding compounds possess superior electrical properties far beyond the next best material under adverse conditions of extremely high temperature and high humidity. They also possess reasonably good tensile strength, impact-resistance, and other mechanical properties. The material costs, although high, are not intolerable for many servicefi. New compounds recently available have eliminated some of the previous limitations, the requirement for long press cure time, and long postmold oven cure. Sticking to the mold hae also been eliminated. SILICONE POLYMERS
All the siloxane polymers are based on an inorganic skeleton of silicon atoms joined by oxygen atoms linked together to form long chain molecules (8, 10, 11)
Although all the siloxane polymers have a similar elreleton, the influences of the chain length and of the other substituents which are joined to the silicon atom have a tremendous effect on their chemical and physical properties. Whole different classes of materials can be formed, ranging from low molecular weight oils through rubberlike elastomers to hard solid resins
(4).
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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The t w o residual valences of each silicon atom may contztin either organic groups or oxygen atoms. When t,here are two organic groups to each silicon atom, the polymer is called difunctional. Such polymers are either oils or elastomers depending on their chain lengt,h. For the oils chain length is regulated by monofunctional "chain stoppers." I n the case of the elastomers, the chain length is usually made as long as possible. METAL HOllSlNG
CAP
INSERT
PLUG
-
METAL HOUSING
CAP
SOCKET CONTACT
The present general purpose elastomers are difunctional cliairis, all of whose side groups are methyl. The three-dimensional polymers, on the other hand, commoiily contain both methyl and phenyl side groups. The relative proportion of methyl and phenyl groups is another major variable in the silicone resim. By proper selection of these various factors, a wide variety of polymers can be obtained. The commercially available resins range by gradual steps from very flexible, almost rubberlike mateiids on^ the one hand, to hard, brittle, glasslike polymers on the other. The methods of curing the two types of polymers arc also very different. In the case of the difunctional polymers, the chain length is established before curing begins. The curing process consists of cross linlcing between adjacent methyl groups on two separate chains t o form an ethylenic bridge by means of an oxidizing catalyet. I n contrast, the molecular structure of the trifunctional polymers continues to grow during the curing proceas. The reaction proceeds by condensation between the hydroxyl groups, to split off water and form oxygen linkages in three dimensions (6',12).
RECEPTACLE Figure 1. Armg-Na\y Connector, Exploded Vie%
When the groupE contain three oxygen atoms per silicon atom they are termed trifunctional. I n such polymers, the moleculai growth can proceed in three directions rather than two, and thereby form a three-dimensional thermosetting resin. Polymers consisting entirely of trifunctional groups closely resemble silica, and form hard, brittle, glaselilte materials on complete curing.
Curing of Silicone Resin Polymer
The most effect'ive resin catalysts are materials such as tri-
PIN W L L
STRENGTH.LB.3 100(
1
MOLDING TIME.MINUTE5 AT 3SO'E
Figure 2. Effect of Cure on Pin Pull Strength of Resin C
The literature of the resin polymers is much less extensive than that of the difunctional elm. Alt3hough many silicone resins are commercially available, their manufacturers do not usually disclose details of their composition. The resins khat are most commercially applicable consist of copolymers of difunctional and trifunctional groups. By this means it is possiblc to modify the hard, brittle character of the strictly trifunct,ional polymers. The chemical type of side groups att:tched t o t,he silicon atoms has a very important effect on the propert,ies of thc polymer.
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k'e
ethanolamine, zinc and iron octoates, and naplithenates, that are widely used in various condensation reactions. The ultimate properties of the cured resin, as well as the rate of curing, are markedly influenced by the catalyst employed. I n ordcr to develop optinium properties in silicone resins, til(, difunctional and trifunctional units must be uniformly djstrihuted throughout the polymer. If it were possible to obt,ain this arrangement, n very thoroughly thermoset resin would result n-ith a built-in plasticizing effect from the difunctional rubberlike substituents. Cnfortunately, this goal is never completely realized since the trifunctional units have a much greater reactivity rate than the difunctional units, and during the course of the polymerization, tend to react with each other to form cyclic. structures, leaving the unrcacted difunctional units complctdy alone. I n the extreme case, the resulting structure con islands of interreacted, cyclicized, trifunctional units in a wa of difunctional chains. Khen t,his happens, t,he benefit of h:ivirig the trifunctional monomers link the entire structure togcther is lost, and a somewhat thermoplastic mixture result,s. This difficulty in controlling the course of t,he polymerization is one of t,he most serious liniitations of the ailicone resins. SILICONE MOLDING COMPOUNDS
Although unfilled silicone resins have becn widely usrd as elcctrical insulating varnishes and protectivp coatings ( 1 , 5,5, 7 , 16') the molding conipounds have been limited in their coriiniercial utilization by several serious drawbacks. First of all, t,hcy have required both excessive time in the mold and postrnold w e n curing, to develop optimum physical properties. In addition,
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46. No. 11
..
-SiliconesPIN PULL STRENGTH.LBS. 1001
1
90
TABLE111. PHYSICAL PROPERTIES AFTER AGINGAT 200' C. FOR
NO O V f N CURE
Compound G B Glass Mineral
80 Compressive strength, Ib./sq. inch Tensile strength. Ib./sa. inch Impact r e s i s h o e , f t . ib./inoh notch. Dimensional change, 720 hr , %
70
60 50
48 HOURS Good Performance
21,374 4,000
13,641
4,000
3,000
0 47 0 37
0 52 0.50
0.50 0.20
10,000
40 30
TABLE IV. PHYSICAL PROPERTIES OF TYPICAL SILICONE MOLDING COIVIPOUNDS~
20
IO 0
Figure 3.
Effect of Postmold Cure on Pin Pull Strength
WT LOSS I Anr
Tensile strength, lb./sq. inch 4000 After 48 hrs. a t 200' C.a linaact strength. ft. lb./inch notch 0.5 After 48 hr. a t 200° C. eompressive-strength,'lb./sq. inch 14,000 t o 22,000 After 48 hr. at 200' C. After 48 hr. at 200° C. Flexural strength 2900 4000 After 5 months a t ZOOo C. 5100 After 5 months a t 284' C. Dimensional stability, % 0.15-0.39 length After 1 month a t 200° C. 0 16-0.69 thickness After 1 month a t 200° C. a Measured a t room temperature after conditioning. b Plus 1 month a t 70' C. and 100% relative humidity.
35 30
MEGOHMS IO9
25 20 15
IO 5 0
KEY
Figure 4.
NO POST MOLD CURE POST MOLD CURE. l6HRS.020O0C
Effect of Postmold Cure on Solvent Resistance
Refluxed in boiling toluene for 24 hours
the thin sections of the molded pieces had very little strength and tended to crack when they were removed from the mold. Furthermore, the pieces frequently stuck to the mold. In the past, high price has restricted their use, but it is felt that the wide scale, commercial use and increasing competition of the entire
TABLE I. TYPES OF FILLERS IN MOLDING COMPOUNDS Mineral Filler Resin
B"C
D
E
F Glass Fiber G
H I
Combination Fiber and Mineral J
Manufacturers Designation GhIG A-5001 G M G -4.5002 C P 701B C P 701T XAI 3 xnr 37 G M G A-5003-Chopped fibers G M G 12432-Long fibers G E 12810-Long fibers
EX 686
TABLE 11. TYPICAL SILICONE MOLDING CO&fPOUND Silicone resin DC 2104 Diatomnoeou$ silica, (2-270 Triethanolamine Strontium naphthenate
November 1954
Per Cent 35 t o 40 6 3 . 5 to 5 8 . 5 1.0 0.5
VOLUME RESISTANCE PERCU. IN.
Figure 5. Effect of Exposure to 200" C. (Dry) for 720 Hours on Electrical Properties
silicone field will materially reduce the price of the silicone molding polymers. The present study has demonstrated that the excessive curing requirements and the molding difficulties have been overcome by recently developed molding compounds and the use of a new mold release agent. The molding compounds differ from the coating and laminating resins in that they contain a major proportion of fillers. Although a great many materials have been evaluated, as potential fillers for the resins, only two types have found acceptance for commercial use. These are the mineral, silica, in one form or another, and glass fibers. The type of filler has an important effect on the properties of the molding compound. The mineral-filled type is easy to mold, but has low impact strength. The long glass fiber-filled type has much better impact strength, but is very difficult to moId, because the long fibers get trapped in the narrow sections of intricate molds. The fluid resin flow away, leaving aIternate areas of high fiber content and high resin content. A compromise in physical properties consists of using short glass fibers, sometimes obtained by milling the molding compounds before use. Table I shows the types of filler used in ten commercially avail-
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MEGOHMS IO'"
provide readily detachable electricai i.driiiei.thii~ for li1ul tipic, eircuits (Figure 1 ). h number of dift'erent designs of AS coiiriectors are available, hut they all consist, basically of a plug and a receptacle aspenibly that contain suitable irieulating inserts in which are mounted pins or sockets that provide the e1ectric:il contacts. K'heii t,he connector components are assembld, they form a rigid unit somewhat on the principle of a pipe union. After the aircraft has been resting on the ground. t,lie air i i i the space between the two halves of tlie connector is inor(' 01' less saturated with moisture a t relatively high temlier:rtun%. \Then the craft becomes air-borne and reaches high altitudc,, the air is chilled below its dw- point temperature, arid moisture. condenses on the surface of tlie dielectric material. This oi'tc'ti nce t o t h e point that zrc.-ovcr rcwilt.. reduces the electrical r One atternpted sol to this problem has h e m t o U S P elastomeric iiisulating materials which can be c>onipr completely fill the void irithin the hardware of the corincxctors thuJ excluding air and its attendant moisture. 'This approarli has been fairly succrssful but has iiitroduced otkiei. prol)lc~ni~. Most elastomeric materials are limitctl iii their tcml)c~ii\turc~ range and have rather poor nrc resistance. I t is not f i w i l i l ( ~to bond tlie pin and socket, contacts to the r u b i ) ~ . wires fastened to the contacts sometimes cnu pulled right out of tlie elastomrric insert.
I
VOLUME RESISTANCE PER CU.IN.
Fignre 6. Effect of Exposure to 100% Kelati,e IIumidity at 70' C. for 720 Hours on Electrical Properties
able siIicone molding compositions which vere evaluated in this study. Suppliers were Bakelite Co.; D o u Corning Corp.; Federal Telecommunications Laboratories; and General I
