POLYTHENE Physical and Chemical Properties J
FREDERICK C. HA”,
MAURICE L. MACHT,
AND DAVID A. FLETCHER E. I . du Pont de Nemours 8% Company, Inc., Arlington, N. J.
N
The hydrocarbon resin, polythene, recently became available commercially from UMEROUS polymers of ethylene domestic sources. The low electrical losses, along with high resistance to have been prepared by many inmoisture and chemicals and toughness over a wide range of temperatures, vestigators by direct polymerization (16‘). have led to wide usage, particularly in the electrical field; these properties These experiments yielded liquid hydroindicate other potential applications. The unusual resistance of this material carbons of relatively short chain lengths. to various chemicals suggests other fields for its use; data show the effects of Early workers synthesized small amounts exposure to various chemicals on the physical properties. They p i n t to such of solid hydrocarbons of greater chain applications as special types of equipment-containers, gaskets, tubing, etc. length by various chemical means, such A s a means of protecting metal surfaces from corrosion and applying polythene as the decomposition of diazomethane coatings in general, the process of flame spraying the powdered material is @), action of sodium on decamethylene described. Certain flame-sprayed compositions show good adhesion to metals bromide (4, and other reactions; but even these products were also of relatively and give excellent protection against corrosion by water, brine, etc. Data are low molecular weight. Subsequently presented on other physical properties of polythene, such as the relation of Fawcett and Gibson started their hisdensity or volume to temperature. toric fundamental studies of the effect of high Dressure on chemical reactions, Its molecular structure, as disclosed by x-ray studies, was thorinclulding polymerization (10). As a result of these researches oughly investigated by Bunn (3). The principal crystalline conFawcett, Gibson, and Perrin (9) were the first to find that it was stituent of polythene represents the simplest type of crystdine possible to obtain solid polymers of ethylene of sufficiently large structure in hydrocarbons. Bunn’s data show the unit cell to be molecular weight to be attractive for use in plastics. orthorhombic, containing four CHt groups, with a fiber-identity “Polythene” is a generic name originated by Imperial Chemical period of 2.53 A. and an average distance between chains of 4.30 A. Industries and adopted by du Pont to define those polymers of This corresponds to planar zigzag carbon chains, with atoms ethylene suitable for use in plastics. The solid or semisolid polyseparated by the usual 1.54 A., and the tetrahedron valence angle mers included under this generic name may be further described of ordinary normal paraffins, as found by Hengstenberg (13) a.s the substantially saturated chain hydrocarbons of which the and others. Its spatial arrangement is best illustrated by the average individual molecules are formed from about 100 or more Hirschfelder model (Figure 1). ethylene units. The term “polythene” includes, therefore, a PHYSICAL. Polythene is essentially a microcrystalline plastic series of polymers of ethylene of various average molecular containing some amorphous material of similar composition, and weights. its mechanical behavior resembles in many respects that of The present paper is concerned chiefly with polythene of the nylon. The arrangement of its microcrystalline components, or type and grade developed especially for electrical uses and having crystallites, may vary considerably according to the source and an average molecular weight in the neighborhood of 18,000to treatment of the sample. I n polythene which has not had any 20,000. This product, like others in the higher ranges of molecuspecial pretreatment, the crystallites tend to occur in a structure lar weight or viscosity, is tough and flexible over a wide range of of spherulitic type, which on x-ray examination gives an over-all temperatures (including very low ones), is translucent, and is noreffect of random distribution of crystallites. It consists largely mally white in color. Its structure results in unusual water reof spherulites which are radial or tangential aggregates of cryssistance and characteristic electrical properties, such as extremely tallites having, under ideal conditions, a gmsa structure of spherlow power factor and low dielectric constant, along with high reical shape. Microscopic evidence suggests the radid internat sistivity and high dielectric strength. The unusual combination structure rather than the tangential, although this conclusion is of electrical and mechanical properties makes polythene suitable not fully supported by the behavior in polarized light. The for the insulation of high-frequency and high-voltage equipment. spherulitic structure is best seen under the polarizing microscope The resistance of this material to water and chemical attack (Figure 2A). marks it as outstanding for waterproofing, corrosionproofing, The size of the spherulites can be varied by thermal treatment. gasketing in chemical plants, containers, and sheeting for packagBy shock cooling, for example, a sheet can be made to contain ing (19, 20,$1). only a few spherulites large enough to be resolved by the ordinary STRUCTURE microscope. Such sheets are more transparent and flexible than ones which have b.een slowly cooled. In slowly cooled films the CHEMICAL.Polythene is composed of long chains of methylene spherulites may measure 20 microns or more. groups, which in the simplest way may be represented structurally If tension is applied to test specimen of sheeting at ordinary as follows: temperature, the elongation during the first phase of the stretching, up to about l00%, is substantidy reversible, and if tension is H H E€ H H H H H released during this phase, the specimen recovers to a large extent -4-c-c-c-c-c-c-cinstantly and then in a short time almost completely. When the H H H H H H H H
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INDUSTRIAL AND ENGINEERING CHEMISTRY
S27
Figure 1. Hirsehfelder Spatial Model of Polyrhene
tensile stress exoeeds B certain cnticd value, a second phose of stretohing OCCUA, up to about 600-750'& in which R permanent orientation of the crystailitw takes place and the specimen necks down as cold drawing iroeeeds. Cold drawing effects a change in appearance of the gross &metwe which may be seen by comparison of Figure 2A with 28. The surface of the unstretehed film shows a roughnws under the miemscope, and there are n ~ m e r o uvalleys ~ hetween the 8pheruliten. In Figure 2B the valleys have been elongated. It would appear that, when the fih is stretched the material in the spherulites is d r a m out into a different arrsnpenent. The rearrangement of the crystallita aod long moleoules in the amorphous portion, into position parallel to the direction of drawing, can be detected hy measurement of birefringence(Figure ZC) or by x-ray examination. Orientation may be effected also hy cold rolling a sheet, and the extent of such orientation may be varied by the conditions of the rolling operation. This general phenomenon of orientation is of grest interest, both scientifically and oommercially. INFLUENCE OF MICROCRYSTALLINE STBUCI'URE ON PHYSICAL
PROPERTIES
The microcrystallinity of polythene appears to be an important fw&r in the chsnges in certsin of ita physical prop erties with temperature, far the rewon thst the microcrystalline condition itself is strongly inEuenced by temperature. As the material is heated from room tanpasture, it reaches a definite range of tampasture within which there is B transition from crystalline to amorphous condition. As this tempersturn rangeis approached, the rate of change of certain physical properties with temperature hecomes grester. This phenomenon is illustrated by curves showing the v~riatiom of refraotive index, density, eoeacient of expansion, and specificheat with temperature. Figure 3 shows the ehanse of index of refrsotion with temperature; the change in this property in the vicinity of 80-100" C. appears to he c s u d hy the disappearance of a certain crystsuine phase w the remit of fusion or eolution. Double refraotion diasppears in this temperstwe mge; however, under the plarieing micmaoope there ia some evidence of persistence of crystalline structure upto*lout 1 W C . Above110-115°C.
the relnt.ion of refractive index tu tmripeitrturc i s a straight line, typical of polymem of liqrridlikc slmcture such as natural rubber and polyisobutylone. Andogous pherioniona are illustrated by the curves showing vnriationii of density ( l 4 ) ,thermal eoemcient of cubical enpansioa, a i d specific heat (I,$) with temperature (Figure 4). The high values for spocifie heat of polythem just below the melting point may he attribut,ed to cit.hor heat of fusion OP heat of soliition---that, is, fusioii i f the liquid phose has the same composition ax t,lm solid, or soluliorr if the two Dhms are not identical.
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It should be mentioned that the values given in this paper for physical properties are typical of polythene but will not apply exactly to every specimen, since the properties are subject to some variation according to the source or type of the polythene, and the method of making the test specimen.
Vol. 37, No. 6
sion effects an irreversible extension, and during this phase the elongation proceeds under nearly constant stress until the specimen has been cold-drawn to the ultimate extent over its whole length. Further application of tension results in a furthcr elongation and then in failure. The elongation produced by
TEMPERATURE- "C. Figitre 3.
Effect of Temperature on Refractive Indices of Polythene, Polyisobutylene, and Natural Rubber
PROPERTIES
Comparison with other plastics shows that MECHANICAL. polythene combines a number of good mechanical properties. As Figure 5 shows, the stiffness of polythene places it in a category between those materials known rn rigid plastics and those termed nonrigid plastics. The stiffness of polythene, in common with that of other well known plastics, varies considerably with temperature. Table I reports mechanical properties of polythene at 77" F. In the cases of some properties in which temperature is of particular importance, variations with temperature will be shown later. The stress-strain diagram of Figure 6 shows that the application of tension to the test specimen causes elongation in a regular manner over the entire length until the stress has reached a critical value. Beyond this point the further application of ten-
cold drawing is of the order of 600-750% in sheeting and of the order of 50% in injection-molded specimens. Its magnitude depends upon the nature of the polythene, the conditions under which the specimen has been prepared, and the conditions under which the drawing is done (e.g., the rate of stretching). The ease with which cold drawing is effected is dependent upon a number of factors such as average molecular weight, distribution of molecular weight, method of fabrication of specimen, and temperature. The tensile strength is, of course, much greater when calculated per unit of cross section of drawn material than when calculated per unit of cross section of undrawn material. The tensile strength of polythene, calculated on the original undrawn cross section, varies from about 800 pounds per square inch at 70" C. to about 5000 at -60' C. (Figure 7). THERMAL.Table I1 presents the thermal characteristics of polythene. The low flow temperature indicates the ease with which this material can be extruded and molded. The heat-
PROPERTIES OB POLYTEENE AT 77" F. TABLE I. MECHANICAL A.S.T.M. Method ProDertv Value of Test 1900 D6384-2Ta Tensile strength, lb./sq. in. 50b; 00O-75Oc 0638-42T"' Elonmtion % 14,600 D638-42T hIodulua oi elasticity. Ib./sq. in. 1.700 D650-41T Flexural strength,, Ib./sq. in. D747-43T 13,jOO Stiffness. Ib./a In. .. ..,. Impsot s t r e n g z (Izod), ft.-lb.,/in. D229-43 Rockwell hardness D624-4lT Tesr resistance Ib./in. of thickness 990/ 0 Shear atrength,' lb./eq. in. * For thin sheet8 tested by method D4112, die C: tenaile. 1700-2400 Ib./sq. in.; dongation, ZO&OO%.. b Injection-molded specimen, 0.25 inch thick. ' Sheeting. Did not break in a 4 ft.-lh. machine. Specimen 0.075 !rich thlok. / Specimen 0.040 inch t h c k . # Johnson ahear jig.
- -
.
TABLE 11.
THERMAL
-~
CHARACTERISTICS OF
P0LYTHEX.E
ProDertv Value 104 Flow temp., a C. 0.6-0 8 Deformation under load, % a t 122O F./15E0 F. yo 105-180 Strain-release temp., F. Yield temp.. F. 140 Heat-distortion temp., low-load * F. 122 Coefficient of linear expansion pbr 0 F. (rang4 7725 X 10-6 122O F.) Coefficient of cubical ex snsion per F (Fig. 48) Thermal c o n d u c t i t t g g.t.u./hr./sq. ft'./'F./in. 2.4 Below -50 Brittleness temp., Specific best (Fig. IC) a Proposed addition to A.S.T.M. D621-41T. b Dealgnation of method developed in authors' laboratory. Proposed revised method of A.S.T.M. D048, 66 Ib./rp. in. d Cenco-Fitch apparatus.
Method of Tast D50!-43 M-15b
M-8;
D096-42T
.. .... .. . . . .
D740-4aT
June, 1945
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE rrr. ELECTRICAL PROPERTIES OF POLYTHENE Property Dielectric stren th. short-time. volts/miii. Volume reaistivkty, ohm-om. Dialectdo constant 60 oyolca 101 oyclea lo( oyolca 1080 o h Power !actor eo cyclel 10' oyclea lo( lo( oyolea cyclea
Value 475 (0.125 in.)
10'1
2.3
2.3 2.3 2.3
