3,227,784
`H. BLADES ETAL
`Jam 4, 1966
`PROCESS FOR PRODUCING MOLECULARLY ORIENTED STRUCTURES BY
`_
`_
`_
`EXTRUSION OF A POLYMER SOLUTION
`Orlglnal Flled Jan. 31, 1952
`
`4 sheets_.sheet 1
`
`F|G.1
`
`0
`
`
`
`POLYMERCONCENTRATION0%
`
`I00
`
`TEMPERATURE ° C
`
`T
`
`NETHYLENE CHLORIDE - LINEAR POLYETHYLENE
`
`F|G.2
`
`
`
`POLYMERCONCENTRATION0%
`
`200
`I50
`TEMPERATURE °C
`
`250
`
`BY
`
`INVENTOR3
`
`HERBERT BLADES
`
`ES RUSHTON WHITE
`
`ORNE Y
`
`Page 1 of 18
`
`A
`
`BOREALIS EXHIBIT 1067
`
`Page 1 of 18
`
`BOREALIS EXHIBIT 1067
`
`

`
`Jan. 4, 1966
`H.BLADES ETAL
`3,227,784
`PROCESS FOR PRODUCING MOLECULARLY ORIENTED STRUCTURES BY
`EXTRUSION OF A POLYMER SOLUTION
`Original Filed Jan. 51, 1962
`
`4 Sheets-Sheet 2
`
`
`
`
`
`POLYMERCONCENTRATION0°/o
`
`TEMP. “C
`
`FIQ4
`
`I50
`
`200
`
`250
`
`BY
`
`TEMP. “C
`
`INVENTORS
`
`HERBERT BLADES
`JAMES RUSHTON WHITE
`
`ATTORNEY
`
`FIGB
`
`0%
`
`
`
`POLYMERCONCENTRATION
`
`I00
`
`Page 2 of 18
`
`Page 2 of 18
`
`

`
`3,227,784
`H. BLADES ETAL
`3311- 4, “:55
`ROCESS FOR PRODUCING MOLECULARLY ORIENTED STRUCTURES BY
`EXTRUSION OF A POLYMER SOLUTION
`
`FLUOROTRICHLORONETHANE - LINER POLYETHYLENE
`
`°/.
`
`
`
`F|G.5 POLYMERCONCENTRATION
`
`I00
`
`TENP. °C
`
`°/o
`
`
`
`POLYMERCONCENTRATION
`
`INVENTORS
`
`HERBERT BLADES
`
`
`
`ATTORNEY
`
`
`Page 3 of 18
`
`Page 3 of 18
`
`

`
`3,227,784
`H. BLADES ETAL
`Jan. 4, 1966
`PROCESS FOR PRODUCING MOLEGULARLY ORIENTED STRUCTURES BY
`EXTRUSION OF A POLYMER SOLUTION
`Original Filed Jan. 31, 1962
`
`4 Sheets—Sheet. 4
`
`FIG. 7
`
`o
`
`3‘
`
`g 20
`E1
`
`40
`
`'5
`
`§3
`
`5 so
`
`I I8 e
`
`n
`
`80'
`
`I00
`
`50
`
`I00
`
`I50
`
`F DEGREES CENTIGRADE
`
` {CVEC‘“
`i SIIIIIIA F‘
`
`
`
`
`
`BY
`
`Page 4 of 18
`
`5
`
`
`
`%
`
`INVENTORS
`HERBERT BLADES
`JAMES RUSHTOH WHITE
`
`ATTORNEY
`
`Page 4 of 18
`
`

`
`United States Patent Office
`3,227,784
`Patented Jan. 4, 1966
`
`1
`
`2
`
`3,227,784
`PROCESS FOR PRODUCING MOLECULARLY
`ORIENTED STRUCTURES BY EXTRUSION
`03? A POLYMER SOLUTION
`Herbert Blades, Wilmington, and James Rushton White,
`Chadds Ford, Del., assignors to E. I. du Pont de Ne-
`rnours and Company, Wilmington, Del., a corporation
`of Delaware
`Original application Jan. 31, 1962, Ser. No. 170,187.
`Divided and this application Feb. 10, 1964, Ser. No.
`354,192
`
`20 Claims.
`
`(Cl. 264—53)
`
`This application is a continuation-in-part of appli-
`actions Serial Nos. 858,725 and 858,772, filed December
`10, 1959 and Serial No. 736,337, filed May 19, 1958
`which are in turn contiriuations-in-part of application
`Serial No. 665,099, filed June 11, 1957 and -all now aban-
`doned and is a divisional application of Serial No. 170,-
`l87, filed January 31, l962.
`This invention relates to a flash extrusion process for
`preparing microcellular and other molecularly oriented
`polymer structures.
`The porous polymer structures which heretofore are
`well known in the art can generally be described as con-
`sisting of a solid polymer matrix having distributed
`therein voids which may be either isolated or intercom-
`municating. The shape of such voids varies all the way
`from spherical pockets to quite irregular channels de-
`pending on their method of preparation.
`Irregular chan-
`nels normally arise, for example, when the porous article
`has been prepared by partial fusion of the surfaces of
`the particles of a granular polymer mass. Extraction
`of a fugitive material (e.g., salt crystals) from a solidi-
`fied mixture of molten polymer containing a major pro-
`portion of said fugitive material has also yielded irregular
`channels.
`Other prior art processes generally tend to give more
`nearly spherical voids. One procedure has been to dis-
`tribute a solid blowing agent throughout a polymer mass
`and heat this mixture above the polymer softening tem-
`perature whereupon the blowing agent decomposes to
`gaseous products which are trapped within the polymer
`to produce voids. Another method has been to whip a
`molten polymer into a coarse froth and cool the same
`to solidification. The bubble size may be controlled in
`part by addition of surface active agents or by extrusion
`through bubble comminuting screens.
`A critical feature in all of the above processes is sta-
`bilization of the porous product before the hot polymer
`collapses and destroys the structure. Various ways to
`improve the form stability of the hot structure include
`quenching in a liquid coolant or lightly cross-linking the
`polymer, either chemically or by irradiation.
`Another problem is control of bubble size. Some con-
`trol has been achieved in the prior art processes by ad-
`justing temperature, blowing agent, or gas concentration,
`rate of expansion, addition of surface active agents, and
`extrusion through screens as mentoined above.
`It has
`also been reported that when fillers are added, smaller
`sized bubbles are produced, although the primary pur-
`pose of the fillers has generally been to increase opacity,
`furnish color, lower cost, or improve strength or stiffness.
`It
`is an object of the present
`invention to provide
`an eflicient process for the direct production of ultra-
`microcellular structures and/or plexifilamentary products
`from a polymer solution. Another object
`is to pro-
`vide a process for producing a supple ultramicrocellular
`shaped structure of synthetic organic crystalline polymer
`. having high strength and relatively low apparent density.
`Still another object is to provide a process for produc-
`ing ultramicrocellular yams and sheets which are uni-
`
`Other objects
`
`form and opaque, even in thin sections.
`will appear hereinafter.
`The process of this invention provides integral utra-
`rnicrocellular products comprised of open -and closed cells
`in any proportion. Open cells are usually formed from
`closed cells whose end walls (i.e.,
`those generally per-
`pendicular
`to the machine direction) have ruptured.
`These frequently occur in sequences, leading to tunnels
`or channels.
`Substantially all of the polymer is present as filmy
`elements Whose thickness is less than 2 microns and pref-
`erably under 0.5 microns. The thickness of a cell wall,
`bounded by intersections with other Walls, does not vary
`by more than :30%. Adjacent walls have equal thick-
`ness Within a factor ‘of 3. The polymer in the cell walls
`exhibits uniplanar orientation and a uniform texture.
`In
`strand form, the ultra microcellular structure has a tenac-
`ity of at least 0.1 g.p.d. The microcellular sheets have
`in general, a tenacity of at least 5 lbs./in./oz./yd.2 in the
`machine direction -and a TAPPI opacity of at least 70%
`at 1 oz./yd?
`is between
`The apparent density of the products (,0)
`0.5 and 0.005 rg./cc. The number of cells per cc. (nf),
`is at least 105, preferably 105 or greater, as estimated
`from the equation
`
`3
`
`_ P
`nf_<35.0o)
`Where t is the Wall thickness in cm., and po is the bulk
`polymer density.
`The wall thickness and transverse cell dimensions are
`determined by microscopic examination of cross sections
`cut perpendicular to the machine direction. Thus 20-60
`micron thick sections may be cut from a frozen sample
`with »a razor blade. Large cell (>50 microns) samples
`are frozen directly in liquid nitrogen.
`Smaller celled
`samples are preferably “imbedded” in water containing
`a detergent and then frozen and sectioned. The trans-
`verse dimension of one or more cells can be readily
`measured by the freezing and sectioning technique 1nen-
`tioned above which at least partially inflates the cells.
`The cells will then exhibit a general polyhedral shape
`as illustrated in FIGURE 8, similar to the shape of the
`internal bubbles in a foam of soap suds.
`It is found
`that
`the average transverse dimension of the cells is
`less then 300 microns and that the transverse dimensions
`of a single cell in a fully inflated condition do not Vary
`by more than a factor of three.
`In the preferred struc-
`tures the average transverse dimension is under 100 mi-
`crons. The ratio of the cell volume to the cube of the
`Wall thickness can be calculated and exceeds about 200.
`For very thin walled samples (<1 micron), the wall thick-
`ness is preferably measured with an interferometer micro-
`scope. A layer o.f the sample is peeled off by contact
`with “Scotch Tape.” The layer is freed from the tape
`by imersion in chloroform and subsequently placed on
`the stage of the microscope for measurement.
`The process comprehends producing other novel and
`useful products as well, by predetermined choice of op-
`erating conditions as hereinafter described. These other
`products comprise fibrous strands of plexifilamentary syn-
`thetic crystallizable polymeric material. The individual
`fibrils of this material are comprised of thin film— or
`ribbon-like strips of polymer which are joined into a
`three-dimensional network. These film—fibrils, generally
`of the order of centimeters in length, often occur in a
`folded T-or crumpled configuration about their long axis,
`accentuating their fibrillar appearance. The fibrils are gen-
`erally coaxial with the strand. Variants of this structure
`occur where the film-fibrils exist in relatively dense com-
`pacted layers connecting portions of the network or en-
`
`10
`
`20
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`30
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`40
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`50
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`Page 5 of 18
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`Page 5 of 18
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`

`
`3,227,784
`
`3
`casing the network in a thin tubular polymeric sheath.
`These plexifilamentary products have certain features in
`common with the microcellular products of this invention,
`namely, the extreme thinness of the film of the structural
`elements and the appreciable orientation of the polymer
`comprising the film.
`The term “uniplanar orientation” employed in defining
`the products of this invention may be fully understood
`from the following discussion.
`“Axial,” “planar,” and
`“uniplanar” indicate different types of molecular orienta-
`tion of high polymeric crystalline materials.
`“Axial ori-
`entation” refers to the perfection with which the crystal-
`line axis parallel to the molecular chain axis in a sample
`is aligned with respect to a given direction, or axis, in the
`sample. For example, prior art materials which have
`been drawn in one direction only (e.g., fibers or one-way
`stretched films) generally exhibit an appreciable degree
`of axial orientation along the stretch direction. Planar
`orientation refers to the perfection with which the crys-
`talline axis parallel to the molecular chain axis is oriented
`parallel to a surface of the sample. Conventional two-
`way stretched films generally exhibit a degree of planar
`orientation in that their molecules lie approximately par-
`allel to the surface of the film, although the molecules
`may point in random directions within this plane. Uni-
`planar orientation is a higher type of polymer orientation
`in that it refers to the perfection with which some spe-
`cific crystalline plane (which must include the molecular
`chain)
`in each polymer crystallite is aligned parallel to
`the surface of the sample. Obviously, only crystalline
`polymers can exhibit uniplanar orientation. There is no
`restriction imposed on the direction of the molecular axis
`Within the plane of the sample. Thus, these three types
`of molecular orientation may occur singly or in combina-
`tions; for example, a sample might simultaneously ex-
`hibit uniplanar and axial orientation.
`Electron diffraction furnishes a convenient technique
`for observing the presence of uniplanar orientation in the
`microcellular structures of this invention. A single cell
`wall is placed perpendicular to the electron beam. Since
`the Bragg angle for electron diffraction is so small, only
`crystalline planes essentially parallel to the beam (per-
`pendicular to the wall surface) will exhibit diffraction.
`If the sample does in fact have perfect uniplanar orien-
`tation, there is some crystalline plane which occurs only
`parallel to the film surface and, therefore, will be unable
`to contribute to the diffraction pattern. Thus,
`the ob-
`served pattern will
`lack at least one of the equatorial
`diffractions normally observed for an axially oriented
`sample of the same polymer.
`If the degree of uniplanar
`orientation is somewhat less than perfect, there may be
`a few crystallites tilted far enough to contribute some in-
`tensity to the diffraction pattern, but at least one of the
`equatorial diffraction intensities will be appreciably less
`than normal. Thus, for the purpose of this invention, a
`sample is considered to have uniplanar orientation when
`at least one of the equatorial dilfractions appears with
`less than one-half its normal relative intensity as deter-
`mined on a randomly oriented sample of
`the same
`polymer.
`One precaution must be observed in making this meas-
`urement.
`If the sample field examined by the electron
`beam is stopped down so far that
`it “sees” only one
`crystallite at a time, it will always be possible, even for a
`randomly oriented sample,
`to find some crystallite ori-
`ented parallel
`to the sample surface which would, of
`course, give a uniplanar orientation diffraction pattern.
`In order to insure that the uniplanar orientation pertains
`to the whole film element and not just to one crystallite,
`the measurement should be made examining a field of at
`least 100 square microns area, which is large enough to
`include the contributions from many crystallites simul-
`taneously. Other techniques of measuring uniplanar ori-
`entation and their correlation with electron diffraction
`measurements are described in the J. Pol. Sci. 31, 335
`(1958) in an article by R. S. Stein.
`
`10
`
`30
`
`05U:
`
`40
`
`60
`
`4
`The term “uniform texture” applied to the polymer in
`the cell walls means that the orientation, density, and
`thickness of the polymer is substantially uniform over
`the whole area of a cell wall, examined with a resolution
`of approximately 1/2 micron. This is best determined by
`observing the optical birefringence in the plane of a wall
`of a cell removed from the sample. For microcellular
`samples with a net overall axial orientation, the individual
`cell walls will also normally exhibit an axial orientation
`in addition to the required uniplanar orientation.
`In the
`birefringence test, such products of the present invention
`will show a uniform extinction over the whole area of
`the cell wall. Samples with no net axial orientation must
`show a uniform lack of birefringence over their whole
`area rather than numerous small patches of orientation
`with each patch oriented at random v/ith respect to the
`others. Lacy or cobweb-like cell Walls, of course, do not
`have uniform birefringence over the whole area of a cell
`wall, and such products are readily distinguished from
`the uniform textured products of this invention.
`In the process of this invention for producing ultra-
`microcellular structures and/or fibrillated (three-dimen-
`sional network of ribbon-like elements) products, a con-
`fined mixture of a polymer plus at least one activating
`liquid is heated to a temperature and pressure at which
`a homogeneous solution is formed, and which tempera-
`ture is greater than the normal boiling point of the liquid.
`(The term boiling point or normal boiling point as used
`herein refers to the temperature at which a liquid boils
`under an external pressure of one atmosphere.) This so-
`lution, either under autogenous pressure or higher pres-
`sure as hereinafter defined, is extruded abruptly to a re-
`gion of substantially lower pressure and temperature
`under such conditions that a very large number of bubble
`nuclei exist at the extrusion orifice. The initial concen-
`tration is chosen, as hereinafter described, such that va-
`porization of the activating liquid rapidly cools the solu-
`tion to the temperature at which the polymer precipitates,
`and freezes in the polymer orientation produced in the
`rapid extrusion and expansion process. These events all
`occur within a small fraction of a second, i.e., 10-2 sec-
`onds or less.
`The use of pressures greater than the vapor pressure of
`the activating liquid (the autogenous pressure)
`is fre-
`quently desired in the process of this invention to dimin-
`ish or eliminate phase separation prior to extrusion, or
`to provide increased nucleation during extrusion. As the
`temperature of the polymer-activating liquid solution is
`raised to temperatures appreciably above the normal boil-
`ing point of the activating liquid, thermal expansion de-
`creases the density of the activating liquid and makes it
`a poorer solvent.
`If the solvent power drops too far,
`phase separation of the solution occurs. This effect can
`be counteracted by applying super-autogenous pressure to
`the system, for example by a mechanical piston,
`to in-
`crease the activating liquid’s solvent power by bringing
`its density back up.
`Separation of the polymer solution into two liquid
`phases may or may not, be desirable.
`If the two phases
`separate readily into two liquid layers of unequal polymer
`concentration, extrusion will be erratic, and the technique
`described above may be employed to produce a single
`phase of uniform composition. On the other hand ex-
`trusion of a two phase system may actually be desirable,
`if the system may be maintained as such a fine disper-
`sion that the dimensions of the dispersed droplets are
`small compared to the size of the extrusion orifice.
`In
`this case the average polymer concentration being ex-
`truded remains constant so that extrusion behavior is
`uniform, and the presence of the droplets will assist in
`achieving adequate fibrillation as described hereinafter.
`A preferred technique with such systems is to employ
`suflicient pressure in the main body of the equipment
`to guarantee a single phase, uniform composition, stable
`solution. This solution is fed through an orifice to re-
`duce the pressure enough to form two liquid phases
`
`Page 6 of 18
`
`Page 6 of 18
`
`

`
`3,227,784.
`
`5
`(with, or without, bubble nucleation as described in a
`subsequent section on “preflashing”)
`just prior to de-
`livery to the extrusion orifice. The step of extrusion as
`recited in the appended claims is intended to encompass
`passage through an orifice of solutions which may be in
`the form of such transitory multiphase systems immedi-
`ately prior to issuance into the region where vaporization
`of activating liquid causes solid polymer precipitation
`and freezing-in of the polymer orientation.
`VVhen superautogenous pressures on the spinning solu-
`tion prior to extrusion are required to achieve adequate
`nucleation,
`these are obtained by dissolving a lower
`boiling additive in the solution. These will assist nuclea-
`tion by increasing the “internal pressure” and lowering
`the surface tension of the solution. Although any solu-
`ble low boiling material is suitable, the preferred mate-
`rials are those which are super-critical at temperatures
`above the polymer melting point. Useful additives in-
`clude N2, CO2, He, H2 methane, ethane, propane, ethyl-
`ene, propylene, certain fluorinated and/or chlorinated
`methanes and ethanes, and equivalents.
`Suitable activating liquids for use in this process should
`preferably have the following characteristics:
`(a) The liquid should have a boiling point at least 25°
`C. and preferably at least 60° C. below the melting
`point of the polymer used;
`(b) The liquid should be substantially unreactive with
`the polymer during mixing and extrusion;
`(c) The liquid should be a solvent for the polymer under
`the conditions of temperature, concentration and pres-
`sure suitable in this invention as set forth below;
`((1) The liquid should dissolve less than 1% of high
`polymeric material at or below its boiling point:
`(e) The liquid should form a solution which will under-
`go rapid vaporization upon extrusion, forming a non-
`gel polymer phase (i.e., a polymer phase containing
`insufficient residual liquid to plasticize the -structure).
`In these requirements, the process of the present inven-
`tion difiers radically from conventional foam producing
`or fiber producing techniques. Choice of a suitable acti-
`vating liquid is, of course, dependent on the particular
`polymer in question. Among those found useful are
`methylene chloride, ethyl chloride, fluorotrichlorometh-
`ane, pentane, butane, and ethanol.
`The polymers suitable for use in this invention are
`members of the class of synthetic crystallizable, organic
`polymers which includes polyhydrocarbons such as linear
`polyethylene, stereo-regular polypropylene or polystyrene;
`polyethers such as polyforrnaldehyde; vinyl polymers
`such as polyvinylidene fluoride; polyamides both ali-
`phatic and aromatic, such as polyhexamethylene adipa-
`mide and polymetaphenylene isophthalamide; polyure-
`thanes, both aliphatic and aromatic, such as the polymer
`from ethylene bischloroformate and ethylene diamine;
`polyesters such as polyhydroxypivalic acid and polyethyl-
`ene terephthalate;
`copolymers
`such as polyethylene
`terephthalate-isophthalate, and eqiuvalents. The poly-
`mers should be of at least film forming molecular weight.
`One of the features of this invention is the high de-
`gree of orientation of the polymer in the cell walls, which
`contributes to the unique strength of these structures.
`Therefore, a preferred class of polymers from which to
`make these objects is that class of polymers which re-
`sponds to an orienting operation (e.g., drawing of fiber or
`films) by becoming substantially tougher and stronger.
`This class of polymers is well known to one skilled in
`the art and includes, for example, linear polyethylene,
`polypropylene, 66 nylon, and polyethylene terephthalate.
`Another feature of the predominantly closed cell micro-
`cellular articles of this invention is their very high de-
`gree of pneumaticity resulting directly from their unique
`structure, which may be looked upon as numerous tiny
`bubbles of gas enclosed in thin polymer skins. Reten-
`tion of this gas, and hence of the structure’s pneumaticity
`
`6
`depends on a low rate of gas diffusion through the poly-
`mer walls. Therefore, another preferred class of poly-
`mers particularly for preparing microcellular structures
`where pneumaticity is important, is that class of polymers
`with low permeability coeflicients for gases, such as
`polyethylene terephthalate. Polymer properties such as
`-solubility, melting point, etc. are usually reflected in the
`properties of the ultramicrocellular product. Common
`polymer additives such as dyes, pigments, antioxidants,
`delusterants, antistatic agents, reinforcing particles, adhe-
`sion promoters, removable particles, ion exchange mate-
`rials, U.V. stabilizers and the like may be mixed with the
`polymer solution prior to extrusion.
`Only certain special combinations of concentration
`and temperature for the activating liquid-polymer system
`will produce the microcellular products of this invention
`and the plexifilamentary materials described earlier.
`Thus, for a given concentration, only a limited tempera-
`ture range is suitable; the minimum temperature corre-
`sponds to the freezing point of the solution, and the
`maximum temperature is limited by the amount of adia-
`batic cooling generated during the expansion process.
`Similarly, for a given temperature, only a limited range
`of concentrations is suitable, a certain minimum quantity
`of activating liquid being required to produce sufficient
`cooling for polymer precipitation on adiabatic evapora-
`tion. To prepare the microcellular products of this in-
`vention, the maximum quantity of activating liquid is de-
`termined by the point beyond which so much liquid is
`evaporated in cooling the solution to the precipitation
`temperature that the resulting cell walls are too thin to
`withstand the residual internal gas pressure whereupon
`catastrophic rupture produces the fibrillated plexifilament
`product. This product in turn cannot be prepared from
`indefinitely dilute polymer solutions, since eventually a
`point is reached where so much gas volume is generated
`per gram of polymer that the structure is no longer co-
`herent, i.e., is not a continuous three-dimensional plexi-
`filamentary structure.
`The permissible combinations of temperature and con-
`centration required in the practice of the process of this
`invention as described above are illustrated in the attached
`figures, where
`FIGURE 1 represents a generalized graphical definition
`of suitable conditions of temperature and concentration
`applicable to any polymer-activating liquid combination.
`The detailed shapes of the curves, as well as the specific
`values of the ordinate and abscissa will, of course, depend
`on the particular system chosen, and may be ascertained
`for any given system as hereinafter described.
`FIGURE 2 represents a graphical definition of suitable
`conditions of temperature and concentration for the spe-
`cific combination of linear polyethylene and methylene
`chloride.
`
`FIGURE 3 represents a graphical definition of suita-
`le conditions of temperature and concentration for the
`specific combination of linear polyethylene and pentane.
`FIGURE 4 represents a graphical definition of suitable
`conditions of temperature and concentration for the spe-
`cific combination linear polyethylene and butane.
`FIGURE 5 represents a graphical definition of suitable
`conditions of temperature and concentration for the spe-
`cific combination linear polyethylene and fluorotrichloro-
`methane.
`
`FIGURE 6 represents an empirical graphical definition
`of suitable conditions of initial temperature and concen-
`tration for the specific combination of polyethylene ter-
`ephthalate and methylene chloride. The area A-BCDE
`corresponds to area C for this system, which will lead to
`microcellular products, as contrasted with fibrillated prod-
`ucts. The boundary line ABC is set by solubility limita-
`tions. The line AGLN is the melting curve, and the
`extension of area C to temperatures below this curve
`illustrates the supercooling phenomenon hereinafter de-
`scribed. Area FHJKM is the preferred operating area.
`
`10
`
`15
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`20
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`30
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`CA301
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`40
`
`50
`
`60
`
`4! C}!
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`Page 7 of 18
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`Page 7 of 18
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`3,227,784:
`
`5
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`10
`
`7
`FIGURE 7 represents a graphical definition of the
`empirical function F~——f(Q) required in determining curve
`III as hereinafter described.
`FIGURE 8 is a photomicrograph of an ultramicrocellu-
`lar structure showing the polyhedral character of the cells.
`FIGURES 9, 10 and 11 are drawings showing cross-
`sectional views of spinnerets suitable for use in the practice
`of this invention.
`Area C bounded by curves I, V, VI, and IV’, of each
`figure defines operable conditions for practicing the proc-
`ess of this invention to prepare the microcellular products
`of this invention. Area A, bounded by curves V, VI,
`and IV’, defines the conditions for preparing the plexi-
`filamentary products. The solution temperature is re-
`presented in degrees centigrade and the polymer concen-
`tration is expressed in weight percents. The individual
`curves are plotted from the following considerations:
`Curve I represents the freezing poirit of homogeneous
`mixtures of polymer and activating liquid at various ratios
`of components. This information is easily ascertained by
`independent determinations on any polymer-activating
`liquid system of interest. Curve I may often be approxi-
`mated satisfactorily by a straight line:
`
`Qfp=gTfp+W
`
`25
`
`(1)
`
`which passes through the two points representing the
`freezing points (Tm) for 10% and 70% mixtures. As is
`well understood to those skilled in the art, a higher order
`polynominal, is generally more representative of a freez-
`ing curve:
`
`30
`
`40
`
`60
`
`Qrp=g1(Trp)2+£'2(Trp)+W
`
`(13)
`
`Qfp is the polymer concentration at the freezing point in
`weight percent, and g and W are constants. All
`tem-
`peratures are in degrees centigrade.
`Frequently, the freezing and melting point curves are
`identical, but for some systems, such as polyethylene
`terephthalate—methylene chloride, substantial degrees of
`undercooling of the solutions are possible so that
`the
`freezing curve (as observed within a reasonable time
`scale) comes at considerably lower temperatures than the
`melting curve. Such supercooled solutions—even though
`metastable—perform satisfactorily, and the freezing point
`curve is thus the proper low temperature boundary for
`area C.
`Curve II is expressed by the equation
`
`T=Tc
`
`(2)
`
`temperature of the activating
`where Tc is the critical
`liquid.
`It is possible to operate at temperatures some-
`what above T0 for those systems having enough inter-
`action energy between polymer and activating liquid so
`that a solution can be prepared even above Tc by applica-
`tion of suitable pressure.
`Curve III may be represented by the equation
`
`T=F+TBP=f(Q)+TBP
`
`(3)
`
`where TB? is the boiling point of the pure activating
`liquid at atmospheric pressure, and F=f(Q) as given
`graphically in FIGURE 7. The function of curve III is
`to define the point (Q’, T’) given by the intersection of
`curve III with curve I.
`(The point Q’, T’ will be em-
`ployed to locate curve IV,
`the sintering curve, as de-
`fined below.) Curve III is obviously simply the curve
`of FIGURE 7 shifted by a number of degrees equal to
`the boiling point of the chosen activating liquid. The
`curve of FIGURE 7,
`in turn,
`is simply an empirical
`curve based on the experimentally determined points Q’,
`T’ for a number of different polymer-activating liquid
`systems.
`It is of value in predicting the approximate
`location of the point Q’, T’ for any system.
`It is that
`Curve IV is defined as the sintering curve.
`special one of the infinite number of cooling curves
`
`Page 8 of 18
`
`8
`represented by equation 4 which passes through the point
`Q’, T’ located by the intersection of curves I and III.
`j 7:4:
`dY
`Cs
`0,,
`dT
`H ' H
`
`(4)
`
`where,
`
`Y=100—Q/ Q
`Cs=heat capacity of activating liquid, calories/gm./ °C.
`Cp=heat capacity of polymer, calories/gm./°C.
`H=the heat absorbed in calories when one gram of
`the activating liquid evaporates from the solution.
`It is equal to H.,+Hi where H., is the heat of vapor-
`ization of the activating liquid in calories per gram
`and H1 is the heat of interaction involved in trans-
`ferring one gram of activating liquid from the poly-
`mer solution into a quantity of pure activating liquid
`at the same temperature. H, is ordinarily so small
`compared to H, that it may be neglected without
`causing serious error in determining curve IV. The
`major effect on curve IV of including H, is to extend
`the curve somewhat above the critical temperature,
`curve II, since I-Iv becomes 0 at the critical tempera-
`ture.
`
`Cp, Cs and H are functions of temperature and may
`be represented in the customary fashion by the following
`expressions:
`
`c,,=a+1;r
`C5=a’+b’T
`
`(4a)
`(4b)
`
`0-as
`
`(40)
`
`r1_[: 2/“+273
`“,,‘:2“73
`H§Hv=H: —%
`L1" T,--273
`a, b, a’, and b’ are empirical constants which may be
`obtained directly from published tables of heat capacities
`or may be individually determined by known methods.
`H1 is the heat of vaporization of the activating liquid at
`a given temperature, T1.
`Inspection of Equation 4 reveals that it is based on
`equating the heat absorbed by evaporation of -incremental
`quantities of activating liquid to the heat released in
`cooling the remaining solution of activating liquid plus
`polymer. This assumption of an adiabatic process is
`justified by the extremely short period of time over which
`the process occurs (less than 10*? seconds). One such
`cooling curve can be constructed starting from any com-
`position and temperature corresponding to a point to the
`right of curve I of FIGURE 1. The existence of an
`unique such curve, curve IV, called the sintering curve,
`can be justified on the basis of the following considera-
`tions.
`If a given composition is heated to higher and
`higher temperatures, eventually a point will be reached
`where even evaporation of all the activating liquid pres-
`ent will absorb insufficient heat to cool the residual poly-
`mer below its melting point. The cellular product thus
`produced would have low form stability, and subsequently
`would appear to be sintered. Thus, in the absence of
`other complications, the sintering curve would be that
`cooling curve passing through the terminal point Q=100,
`T=polymer melting point. However, in order to freeze
`in the polymer orientation generated in the rapid extrusion
`and expansion process, enough activating liquid must re-
`main at the instant the solution reaches the freezing curve
`so that its vaporization will absorb the heat of fusion of
`the polymer and “set” the structure in solid form. The
`lower boiling activating liquids, i.e., those which boil at
`least 60° C. below the polymer melting point, will in
`general evaporate fastest and thus prevent the relaxation
`of this orientation most efliciency, and it is for this reason
`they are preferred. Furthermore, due to the extreme
`rapidity of the expansion process, there is insuflicient time
`for all
`the activating liquid present
`to diffuse to the
`bubble surface and evaporate (it is well known that diffu-
`sion rates become much smaller at lower solvent concen-
`
`Page 8 of 18
`
`

`
`3,227,784
`
`9
`trations), so that in practical cases only part of the acti-
`vating liquid can contribute to the initial adiabatic cooling
`process. Therefore, the practical sintering curve passes
`not through the polymer melting point, but through some
`point Q’, T’ on the freezing curve where Q’ is less than
`100%