T
HE HYDROCARBON RESINS which ph4sessidea!dielectric properties generally fall into two broad classes: (a) Materials like polyiaobutylenewith a second-ordertransitiontemperature below -50' C. A t norial temperatures of usage these pbtiCS are rubberlike, weak mechaniody, and lacking in dimensional stability. (b) Materials such as polystyrene with a sacondorder tramition temperature above '+50" C. At normal temperatures of usage (i.e., below the transition point) such materials SbOW good dimensi.onal stability and high tensile Strength, hut tend to be brittle. The term "Becond-order tramition poiut" is used t o degignate the temperature, or temperature interval, below which bigh polymers behave &B brittle, dimensionally stable materialc;, and above which they exhibit cold flow and rubberlike behavior. For m y plastics the second-order transition point, the heat distortion temperature, and the brittle paint may be within a few degrees of one another. There are exceptions to this generalization. A recent review article (9)discusses this subject more completely and cites pertinent literature references. Polyethylene (10, 18) actually is a combination of both types a and b'since i t is a partially crystalline plastic whose amorphous regionshave a transition temperature of -6OPC., whileits crystal-
line regions are reasonably stable up to f100" C. It thus combines ideal electrical behavior with moderately good mechanical properties. Complications arise here from a high coefficient of thermal expansion and a tendency toward cold fiow. Both of these defects are typical of the weak binding forces between para f f i chains. Oriented polystyrene likewise constitutes an exception t o the general rule from the standpoint of impact strength, Highly oriented monofilaments of polystyrene have heen'made with elongations up to 50% and with tensile strengths as bigh as lM),Mw) pounds per square inch, all with no sacrifice in dielectric response. Doubly oriented sheeta of polystyrene have been molded with a technique developed by the Plax Corpcration into mechanical structures possessing excellent impadt strength. A parallel development using extremely fine oriented polystyrene fibers will he discussed later in the section on radomes. The disorientation tendency of such structures will lower the upper safeworking temperature by as much as 10' C. below that of polystyrene. Mechanical mixtures of polystyrene and polyisohntylene also d o r d a workable compromise between electrical and physical behanor. (Some eight d i h e n t cable companies were engaged in
Plastic c o m p o s i t i o n s for dielectric plastic materials, odifications of the edbtin
the manufacture of polystyrene-polyisobutylene cable formulations until polytbene hecame available.) The exact mode of action of these mixtures is not well understood, although the following explanation might be suggested: Since polylsobutylene is above ita second-order transition temperature, it behaves as a true rubher-that is, as a material whose modulus of elasticity increases m direct proportion to the absolute temperature (0). Below ita transition point, polystyrene behaves as a brittle did-that is, 99 a material whose modulus decreases with increasing absolute temperature. Apparently this interplay of two opposing tendencies is responsible for a product exhibiting satisfactory flexibility and resistance to cold &w over quite a wide range of temperatures (almost to
+lOoOC.). Thus in these three cases of nonpolar hydracarbon polymers or theirmixtures, it has been possible to find at least partial solutions to the fundamental problem. Among the polymers containing polar groups, perhaps the most intereeting caae is that of 2,E-dichlorostyrene (4 12). Here the balanced chlorine
,
groups give risk to a negligme dipole mombnt and, hence, to excellent dielectric properties. At the same time these polar groups 'exhibit stronger intermolecular forces-than those in polystyreue snd, hence, produce a higher second-order transition temperature, Here again normal temperatures of usage are below the transition point and, therefore, in a region where the material behaves &9 a brittlesolid of low impact strength. With polar polymers in general, Le., polymethyl methacryIate, unplasticized polyvinylchloride, etc., the transition temperatures are high (greater than 50' C.). For normal usage, below the transition, the dipoles are frozen in place andnot freetorotatewith the applied field. For such materials, particularly at high frequencies,, theloss factor is only a fraction of the maximum loss factor which that plastic cau develop at elevated temperatures where the dipoles are free to rotate. Polyvinylidene chloride constitutes an interesting exception for a polymer with polar groups. Like polyethylene it has both amorphous and crystalline regions. The amorphous regions have a transition point asoind -10" C. For normal temperatures of usage, the dipoles in the amorphous regions can rotate and give rise to dielectric loss. However, the dipoles in the crystalline regions of the polymer are apparently not free t o rotate (1). Hence the total dielectric loss in t.he
1 .
1
ita rate of polymerization is about seven times greater than that of styrene. Still other salutious and compromises have been developed to overcome this problem. These, then, are some of the general considerations involved in the various attempts to devise and formulate the specific plastic products which will be discussed in the following sections. I n nearly every case some trick was employed, and even then a compromise between physical and electrical properties resulted. The examples that follow &re typical of many which occupied the attention of reseaxch groups at The Dow Chemical Company during the war. Without a doubt, other plastic manufacturers had m n y similar problem. The following items thus exemplify some of the work done hy American plastic groups in extendiug important dielectric frontiers.
Potting mmpannds One of the emliest plastic problems of the recent war concerned the need for potting compounds. This need arose when the invasion of China threatened the supply of tuog oil which formed the base for many potting compounds. A potting wmpouud, or more generally a-casting resin, may be d&ed as a reactable liquid mixture wbkh is stable against storage or shelf life for
.
applications sample is decreased by the presence of crystallinity. The net result is that polyvinylidene chloride combmes tough=, strength, and relatively low dielectric loss at ultrshighfrequencies. This same basic interplay between chemical structure and dielectric properties also hewmes important in terms of curing rates for casting and laminating resins. Production-line techniques and 'other considerations demanded that the time of polymerization be ae short SE possible and still give a finished product of excellent e l e c t r i d characteristics. A fairly general rule is that those monomers which lead to good electrical polymers are the most d@ i c.dt t o polymerize. Ethylene requires high temperatures and extreme pressures; isobutylene requires very low temperatures and active catalrsts. Styrene is relatively easy to polymerbe compared with ethylene, and yet ie slow compared with methyl methacrylate or monochlorostyrene. Dichlorostyrene gave a partial solution in that
Proximity fuse housing, injection-molded with metallic inserts from Ethocel 4 3 0 3
W. C. Goggin and R . F. Boyer THE W W CHEMlCAL COMPANY. MIDLAND, MICH.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1092 10
8.0 6 .o
I
1
I
!
I
I
I
I
I
-- +
,OIL-L-J--J 10'
I
IO'
105
FREO-EhCY, CYC-ES
IO'
109
PER SEZCF.0
Figure 1. Pow-er factor as a function of frequency at room temperature for some of the materials described (measurement supplied by .$. yon Hippel)
several months, fluid enough to be poured into cavities containing electrical equipment, and curable with the least possible volume shrinkage in a reasonably short time, a t moderate temperatures. The cured product must be tough down to -40" C., be nonflowing a t elevat'ed temperatures, and have reasonably good dielectric properties and moisture resistance. Styrene monomer suggested itself a t once as a material which could be readily handled, possessed a reactable double bond, was a good solvent for plasticizers and inert materials, and n-ould lead naturally to cured resins of the desired electrical properties. HOT\-ever, styrene alone was not sufficient because of four characteristic properties: (a) The curing rate is too slow; ( b ) the volume shrinkage during polymerization is too great (177,) ; (c) the cured polymer is too hard; (d) polystyrene is a thermoplastic. Soft vulcanized rubber afforded an excellent example of what was desired as an end product. With this in mind, suitable hydrocarbon type plasticizers were added to styrene to lolver the brittle point to a t least -40 O C., other inert fillers were employed t o reduce the over-all volume shrinkage t o about S%, and several per cent of divinylbenzene monomer was added to function as a crosslinking agent which would raise the flow temperature. This divinylbenzene also functioned to decrease the curing time, possibly in line with the observation of Korrish and Smith that polymerization proceeds at a greater rate in a gel phase ( I d ) . However, the curing time a t 75' C. was still a mat'ter of several days rather than a few hours, even with benzoyl peroxide as catalyst. This was still a relatively nonreactive hydrocarbon system which. could not be expected to cure rapidly. Numerous experiments were conducted with such active cat-. alysts as anhydrous tin tetrachloride and boron trifluoride ( I S ) . The curing rate could be reduced t o a few minutes and even the heat of polymerization was reasonably well controlled by the presence of inert fillers. However, the curing behavior was erratic, presumably because of minor variations in composition and impurities such as moisture content or even atmospheric humidity. These catalysts were finally abandoned. Exterisive research eventually led to a simple organic reaction which reduced the curing time to 8 or 10 hours at 75' C., n-ith no observable impairment of dielectric properties.
Vol. 38, No. 11
Parallel with t'his work, further experiments were carried out on resinous reaction products of styrene monomer with unsaturated alkyd resins. Such reactions had been studied in these laboratories several years before, and have also formed the basis of literature and patent references (8,4, 7, 14, 16). Intrinsically, such resins suffer from the following characteristic properties of the ester linkage: poor electric behavior, affinity for moisture, susceptibility to fungus attack, and hydrolytic cleavage by acids. Counterbalancing these defects, the alkyd can contribute four important functions: It acts as an inert filler to reduce volume shrinkage, it confers toughness a t low temperature, it greatly accelerates the rate of cure by some type of copolymerization with styrene monomer, and finally, this copolymerization gives a crosslinked resin reasonably resistant t o flow- a t elevated temperatures. Through careful choice of ingredients it was possible to design a cured resin of the styrene-alkyd type which fulfilled most of the desired functions. However, the resulting power factor, ranging from 1 t o 49, (Figure I), limited the general application of this material. The present tendency is to return t o the low cost hydrocarbon systems whose polymerization rate can now he controlled with suitable catalysts. One persistent difficulty with any styrene-base casting resin is a tendency to attack bare copper wires. Aldehydes which form in the monomer during storage apparently react with the copper to form oil-soluble metallo-organic compounds which inhibit polymerization. Since p-tert-butylcatechol, the storage inhibitor normally added to styrene monomer, reduces aldehyde formation, it was necessary only to increase this inhibitor content slightly and to protect the liquid casting resin against air as much a i possible in order to reduce copper attack. In general, this casting resin problem opens up a wide field of possibilities. One typical example, created by the wartime shortage of electronic equipment, might be cited. A 10,000-volt tranaformer was needed rather quickly to operate an electronic device. The only available transformer had a peak operating voltage of 5000 volts. This transformer was potted in an electrical-grade casting resin and has been operating ever since at 10,000 volts output. Other electrical equipment, such as coils and transformers, has been protected with such resins againht high humidity or continuous underwater operation and even underground burial. Figure 2 shows several coated transformers. In applying these resins, ingenuity and general economic factors determines hon- important casting resina can become (Table I).
Iladomes The rapid development of special radar equipment during the early stages of the u-ar made it apparent that a parallel development covering radar antenna housings for aircraft would be necessary. Early work in this field was based on molded plywood Fvhich, when dry, appeared to have satisfactory strength characteristics. However, it became necessary to use much of this equipment in humid climates. The moisture pickup of plywood moldings was so great that the radar antenna housing (radome) lost its electrical transparency and rendered the equipment inoperative. The Radiation Laboratory rapidly began to study new materials and nen- methods of fabrication. At this time a request was received coiei,ing the problem of developing materials ivhidi
TABLE I. CHARACTERISTICS OF A HYDROCARBON-TYPE CASTING RESIN(Q-344) PERFOR>l.ASCE OF
PROPERTS Speed of cure a t 50' C. Volume shrinkage Low temp. impact Cold flow Volume resistivity Dielectric constaut Poa.er factor
Q-344
REQUIREYEST
2 t o 4 hrs. Small as possible Tough at -40' C. Stable a t 7 S 0 C. 10" ohm-cni o r greater Small As close to pulystyrene as possible
19 t o 16 h r .
S-9% Satisfxctors Stable a t 100' C . 10'~'o h u - c n i . 2.5
0 lO!L
or less
INDUSTRIAL AND ENGINEERING CHEMISTRY
November, 1946
TABLE11. FUNCTIOKAL PROPERTIES OF RADOMES PROPERTY Fabrication
REQUIREMENT Largesizes a n d quantities must be produced with limited equipment Compressive strength N u s t withstand modera t e s t a t i r load Impact strength Moderate Water absorption Dielectric constant Electrical power loss Production cost
Lon
