CA1092311A

CA1092311A - Extrusion of cellular thermoplastic material

Info

Publication number: CA1092311A
Application number: CA260,150A
Authority: CA
Inventors: John C. Miller; Archibald L. Burnett; Leonard S. Scarola
Original assignee: Union Carbide Corp
Current assignee: Union Carbide Corp
Priority date: 1975-09-04
Filing date: 1976-08-30
Publication date: 1980-12-30
Also published as: JPS5237974A; JPS5545020B2; DE2639814A1; FR2322719A1; DE2639814C2; FR2322719B1; GB1564410A

Abstract

METHOD AND APPARATUS FOR THE
EXTRUSION OF CELLULAR THERMOPLASTIC MATERIAL

ABSTRACT OF THE DISCLOSURE
The invention provides method and apparatus for the production of cellular thermoplastic material compris-ing an extruder having a hollow barrel, screw means mounted in said barrel to advance material therethrough, mixer head means mounted in said barrel and gaseous blowing agent inlet means positioned in said barrel near the upstream end of said mixer head means, said mixer head means having in the surface thereof a plurality of alternate land and groove means, said land means having close clearance with the internal walls of said barrel near the upstream end there-of in the region of said blowing agent inlet means and alternately spaced and close clearance with said walls in the mixing region downstream of said blowing agent inlet means.

S P E C I F I C A T I O N

1.

Description

10,507 1 3 ~ ~

,., The pre~ent invention relates ~o me~hod and apparatus for the production o~ cellular thermoplastic material.
' ~ Heretofore, many ~ystems have been suggested for the productlon of ceIlular thermoplastic material.
However, it has been ound for a number of applica~ion~
that the degree of uniformity o blowing agent disper~ion in the thermoplastic material is highly critical for , successful utilization.
In the production of cellular thermoplastic material in an extrusion process, extrudate variability in unformity is the direct result of such non-uniform r~ `-' dispersion. The latter is critical in many electrical applications, such as the production of insulation for ~- CATV cable.
~: Coaxial cables currently ~mployed to transmit - television signals (CATV cables) predominantly employ ~-;
; cellular low density polyethylene as the insulating material. Processes for the production of such cellular insulat~on are presently employed, wherein production is effected by melting solid thermoplastic material in an extruder into which a gaseous blowing agen~ is injected.
- CATV cables must have a very uni~onm composi~ion ,~ and diamet:er or a portion of the signal may be lost. This ` ~ype of loss is referred to as structural return loss ..
(SRL). Non-uniformities whIch cause 5RL can be due to variation in the extrusion line~ e.g. an eccentric pulley, or in ~he e~rusion process, e.g. ex~ruder surging.
Recent runs have shown that gas injection using conven-,1, ,~ :~

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tional screws can result in an uns table extrusion.
Extruder instability may be measured by fluctuatio~s -~
in head pressure which accurately reflect fluctuations in output or viscosity as shown by the folllowing ` equation: .
Pre~sure = Viscosity x Constant x Ou~put : Thus, measurement of head pressure may be employed as a measure of extruder stability.
. It is, accordingly J the prime object of the present invention to provide method and apparatus for the produetion of cellular thermoplastic material having high degrees of structural and co~positional unifonmlty.
. Other aims and advantages o~ the present inven- ~`
tion will be apparent from the following description and .
` appended claims. :
It is, of course, to be understood that the term hermoplastic material", as employed herein, is well know~ to those skilled in the extrusion molding art and ; ~:
., -- includes by way o~ example such resins as polyethylene, ; :
pol~propylene, polystyrene, polyvinyl chloride and a wide ~ariety of other synthetic organic resinous materials~:
which are accepted as exhibiting thermoplastic properties, together with thermosetting and elastomeric resinous . . .
`~ ma~erials which also exhibit such thermal flowable compo-sitions. In addition, the resins may contain various chemical and/or physical property modi~iers or additives :~ : such as are well known to thP art. ~ :
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10,507-1 ~0~3111 Expanding or blowing gas employed in the apparatus of the invention should pr~ferabIy (especially i for wire and cable applications) be chemically inert toward, and preferably solllble in the base polymer of ~he expandable composition and would thus include inert gases, such as nitrogen, argon, heIium, neon and the like.
.~- Other blowing agents may be used. Ihu~, such blowing agents would include the fluorocarbon type blow-ing agen~s.
~While the use of nucleating agents is not required in the broadest aspects of the pro~ess of the present invention, it has been found preferable to employ nucleating agents.
m e nucleating agents which may be used in the i composltions of ~he present invention are material3 which ~ :~
. provide fine particle sized nucleating sites in the thermoplast~c material base polymer during the expansion or blowing thereof, as described bel~. -: . The particle size of the nucleating agents :~:; 20 should be of the order of about 0.01 to 50 microns. Such . materials would include polytetrafluoroethylene, azodicar~
-~ bon~midej p,p'-oxybis(benzene sulfonyl hydrazide), tri-.. .. .
hydrazino-sym-triazin~, and p-toluene sulfonyl semi-carbazide.
.` The ~ucleating agents may be used individually ~:
. or in eombination thereof.
~j The nucleating agents should be dispersed as :
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uniformly as possible throughout the mas~ of the base ;~ polymer.
~ In accordance wit:h one aspect o~ ~he invention, `. apparatus is provided for t:he production o~ cellular ; thermoplastic material com~rising an extruder having a hollow barrel, screw means mounted in said barrel to . ad~ance material therethrough, mixer head means mounted in said barrel and gaseous-blowing agen~ inlet means positioned in said barrel near the upstream end of ~aid mixer head means, said mi~er head means having in ~he surface thereof a plurality of alternate land and groove ~:
mea~s, said land means ha~ing elose clearance with the ~`. internal wall or walls of said barrel near the upstream .-. end thereof in the region of said blowing agent inlet : :~mea~s and alternately spaced and close clearance wi~h said walls in the mixing region downstream of said blowing ;:
: agent inlet means.
In the drawings~
; :
:~ Fig. l is a cross-sectional view of extruder . 20 and wire drawing apparatus embodying the invention;
- - Fig. 2;1s an elevational view of mixer head i means emp1a~ed in the embodiment of Fig. l;
. Fig. 3 is a sectional-view ~aken along the line ~:

3- 3 of Fig. 2; :
:
Fig. 4 is a sectional view taken along the line

4-4 of Fig. 2;
;. ~
i Fig. 5 1~ a cross-sectional view of a modified extruder embodying the inven~ion; and ~ 1 :
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Fig. 6 is a cross-sectional view of another , modified extruder embodying; the invention.
~; Referring specifically to the embodiment of ' ~ Figs. 1 and 2 of the drawings,extruder 10 i9 provided having outer housing 12, sc:rew means 14 and break~r plate . 16. Cylindrical mixer head means 18 is positioned on ~ the terminal (downstream) end of screw means 14 and com-.~ - prises an internal axial passage 20 and a plurality of ~ - external longitudinal groove means 22 op~n on the upstream `ij 10 ends 26 and closed on the downstream ends ~4.
. Mixer head means 18 is secured to screw means 14 ~: as by scr~w threads 30.
~i As thermoplas~ic material is fed to extruder 10 :
it passes through the screw section where serew 14 is ~:
designed to flux and melt the material before passage to :~ the mixer head means 18. The molten material is divided ~.
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;- . into a pLurality of parallel flow streams whieh pass into.: the plurality o longitudinal groove means ~2 ormed t' be~ween longitudinal land pairs 32 and 3~.
. 20 Gaseous blowing agent i~ introduced into , . ~hEL æ truaer-;:L0 thraug~ ~nlêt 4a which is positioned to inject ~he gas in~o the groove means 22 o he mi~er head 18 near ~he upstream ends thereof. As shown in Fig. -;~
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3, the mixer head lands in this region 4~ are in close '~ i clearance with the inner walls of the extruder:barrel.
~ The rotation of the screw means 14 including the mixer ., ,~ .
head I8 causes the lands to constantly sweep the gaseous ~;; blowing agent-inlet port and intermi~tently open and close ~ ~ passage of gas under substantially full inlet pressure to ''' ' " ,, .

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the parallel molten material stream~ passing through the plurality of groove means 2Z.
. The lands downstream of the region 42 comprise land pairs 32 a~d 34 of different heights, as shown in i. , Fig. 4. As there shown, the leading edge 32 of land pair~
are in spaced clearance with the walls of the extruder barrel to provide therebetween regions of high shear mixing as the material and gas pass ~rom the plurality of grooves over these edges to radial conduits 36 to the internal axial passage 20. The parallel streamis of ~ixed gas and molten thermopla~tic material are there ~oined in ~ .
passage 20 and pass as a co~fluent stream rom the mixer head 18 to discharge chamber 44 upstream of breaker plate 16. Conical member 43 is posltioned at the upstream end of passage 20 to reduce ~he degradation of material in . .
that region.
~ ~.
,~ , The gas and molten :material in the plurality of~
' grooves 22'are there mixed by a circular, swirling action ' ' caûsed by rotation and passage along the inte~al walls of '~ ~ 20 the extruder housing. This action alqo prevents clogging :, of maeerial in the mixing grooves.
The close clearance of the trailing por~ion o~
, the land pairs 34 causes a cleaning action along ~he -- inter~al walls of ~he extruder barrel. :
The molten material discharging through'the breaker plate 15 of extruder lO passes through'chamber 44 ~ .
to die head 46.. ~n eIectricaI conduc~or 48, which is to ~, be'coated with'ceIlular thermoplastic material is fed to the'd~e head at speeds of ~rom about 20 to lO,000 feet per ~ :

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mlnu~e. A uniform coating of molten extrudate is contin-uously applied to the conductor as it passes through ~he die head. The initial thickness of the coating ls deter-mined by the orifice ln die 50 which is located in the side of die head 46, and through which the coated conduc-. tor emerges from the dle head~ The coating thereon beginsto expand because o pressure and temperature and i9 cone shaped for a short distance from the die, but then will stabilize at its ma~imum expanded thickness to yield the uniformly thick ceIlular thermoplastic material coatingaround the electrical conduetor.
As shown in ~he modified embodiment of Fig. S
of the drawings, an extruder 10 is provided having posi- .
- tioned in the hollow internal passage 20 of mixer head 18 means 45 or the prevention andlor disruption of flow ` channeling of the thermoplastic material-gaseous blowing - agent mixture within passage 20. Such means 45 æ e .. secured to breaker plate 16 and extend back toward theupstream end of hollow internal passage 20 to provide a ..
s~ationary agitator assembly 46 having agitating fins 47 ~ :
positionecl around the periphery thereo. These ~ins 47 ;
~- are arranged in a helical pattern, as shown in the embodi- ::
ment of Fi.g. 5. The effect of means 45 and 46 with appended fins 47 is to cause the thermoplastic material-gaseous blowing agent mixture-passing through hollow . internal.passage means 20 to be con tantly sub.jeeted to a mixing ac~ion which prevents ~nd/or disrupts the fl~w ~ channeling of this mixture wit~in the hollow passage 20-.
.. The stream is then passed through an outwardly flaring ~ ~.

10,507-1 1~ ~ 23 ~ ~

passage 48 to an annular m~mifolding region 44 from which it i5 passed through the passages of breaker plate , ~ 16 which are aligned in a x ing around the outer periphery of the breaker plate.
.~ Another modificat:ion of apparatus sui~able for employment in the practice of the ln~ention is sh~wn in Fig. 6 of the drawings wherein a modified means 45' for the prevention and/or disruption of flow channeling is ~5., ' employed comprising aseries of ribbon-shaped spiral sec-~. 10 tions 47' which are axially mounted at right angles to r, ' each other alon~ the ring of ~he hollow internal passage ~-: 20 of the mixer head 18 and are secured ~o the mixer :~ head by rod member 49 at the upstream end thereof. I~
... .
this modified embodiment, agitating action of t~e ribbon shaped means 45' acts to prevent and/or di:srupt the flow channeling oE the mixture of thermoplastic materiaL and gaseous blowing agent passing through the hollow internal ~;- passage 20 and, in turn, through manifold passage 44 and ~3 breaker plate lÇ. -,~ 20 Employing a 2-lf2", 24/1-L/D ratio ex~ruder, .~ various screw designs were evaluated with ~as injection.
,, .
. Extruder stability was measured by head pressure fluctu-ations, w~ich were recorded on a strip chart~ With some -.
designsJ the screw was pushed out after the polymer was - solidified, revealing the 10w ~nd dis.persion of ~he gas.
^ The.results are summarized in the following tableO
`, - In all runs the basic screw used was a conven-, tional polyethylene extruder screw having the following ~, design: .
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Faed Section 3 turns 0,450" depth Transition Section 13 turns Metering Section 4 turns 0. 095" depth Pitch = Diameter (2.5"~
The downstream end of the screw was threaded 50 that vari ous designs could be added making a total L/D ratio of about 23/l. The extruder barrel was drilled to permit - gas injection at ~any locations.
;~ In Run 1, the added end design was a mixing section preceded by a ring. The mixing section had a standard LeRoy type (U.S.P. No. 3,486,192) fluted (grooved) design with three infl~w flutes and three out~low flutes. -~
This section provided mixing and shearing o~ the gas-polymer mixture. The ring provided an annular space or unflighted section into which the gas could be injected.
- It was thought that the absence of a flight would prevent the gas~flow from being cut off momen~arily, and provide :~
- a more stable extrusion. me resulting short term pressure fluctuations were good, but a portion of the gas tended to collect in the annulus section un il saturation leveI was reached, then it would rapidly discharge from the section and exi~ the extruder, in some cases completeIy undispersed.
~his was continuously repeated on a 5 to 10 minute cycle, . .
c~nd the ~oam density varied greatly.
~- In Run 2, the screw end was a section with 12 longitudinal 1utes. For purposes o~ differentiation, this is called a fluted non-mixing section. In this R~n, it served o~ly to con~ey the resin from the end o the screw to the e~truder head. The gas was injected four : . :
::, ' ' 10. ~ i , .

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10,507-1 ~Z31~

~urns upstream of ~he flutes and represented in~ection into a fllghted section of a conventional screw. The buildup-discharge cycle of Run 1 was no~ observed, appar-, ently due to the flight breaking up the gas once per i revolution. However, ~he stability was not good, and observation of a screw pushout revealed that some o~ the , - gas tended to collect and flow along the low pressure side (trailing edge) o~ the screw flight. This resulted in ' poor mixing and accoun~ed for the head pressure ~ariation.
Run 3 employed the s~me screw as Run 2, but the gas was in;ected at the upstream end of the fluted-non-!
mixing section. This resulted in better stability than Run 2 even though there was much less extruder residence time for the gas/polymer mixture. mis showed the impor-tance o~ breaking the gas into small discrete volumes.
In Run 4, the fluted non-mixing section was replaced by the fluted mixing section described in Run 1.
This pro~ided better stability than the fluted non-mixing section despite the fewer number of fLutes. This is .
attributed to the mixing and shearing providPd by this LeRoy cype fluted section.
In Run 5, an internal ~luted mixin~ section having sia ~lutes of the embodlment shown in Figs. 1 and 2 of the drawings was used. Gas was in~ec~ed at the upstream end of the flu~s. The gas/polymer mixture 10wed down the flutes, over a high she~r barrier, through a slot in~o the center of the section, and exited from the ~, center. This design provided excellent stab~lity. It performed better than the mixing sec~ion of Run 4 because . .

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10,507-l J
it had more in flow flu~es and thus could break the gas .. into smaller vol~nes, and addltionally b~cause it provlded '. added residence time while the resin slowly flowed through the cen~er o the section.
,r~ ' Run 6 employed the same screw and mixer head .~: section as Run 5 9 but gas was noc inj ected. This is a control to illustrate that gas in; ection daes reduce ri extrus ion s ta~bility .
,r ~ In these runs, the blowing agent employed was nitrogen gas, the extruder had a screw speed of 80 RE~M :
and an output rate of 80 Lbs/Hr. The resin was poly-ethylene having a density of 0.320 g/cc and a melt inde~
of 0.1 dg/min.

T~
: Extrusion . S tability ~, Foam (Head Pressure ` Location of Density Fluc~ io~s) . Run Iniecti~n Port ~
~-.. 20 1 Annulus (unflightedvaried + l. 0 . . section) . ~ -i~ :" 2 Four turns upstream0.45~ + 3.0 of fluted section i -- tiniection intcJ
: ~ flighted section~
:. 3 Upstream end of 0.45 ~ 2,0 ,- ~ fluted section 4 Upqtream end of 0 . 50 ~ 1. 5 fluted mixing section ~- 5 Upstream end of 0.42 ~ 0,5 fluted mixing section . . 6 ~o gas injected 0. 92 ~ 0 . 25 : ., " ' .
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Claims

WHAT IS CLAIMED IS:

1. An extruder having a hollow barrel, screw means mounted in said barrel to advance material there-through, mixer head means mounted in said barrel and gaseous blowing agent inlet means positioned in said barrel near the upstream end of said mixer head means, said mixer head means having in the surface thereof a plurality of alternate land means and groove means, said land means having close clearance with the internal walls of said barrel at the point of positioning of said blowing agent inlet means so that said blowing agent inlet means is intermittently opened and closed by said land means to the passage of blowing agent to said plurality of groove means, and land pairs alternately in spaced clearance and close clearance with said walls in the mixing region down-stream of said blowing agent inlet means.

2. An extruder in accordance with claim 1 wherein said mixer head means has hollow internal passage means positioned therein over a substantial portion of its terminal length, and a plurality of inlet conduits severally communi-cating between said land pairs of said plurality of land means and said hollow internal passage means.

3. An extruder in accordance with claim 2, wherein means for the prevention and/or disruption of flow channeling are positioned in said hollow internal passage of said mixer head means.

4. An extruder in accordance with claim 3, wherein said means for the prevention and/or disruption of flow channel-ing is mounted so as to be stationary with respect to said extruder barrel.

13.

5. An extruder in accordance with claim 4, wherein said mixer head means rotates, and said means for the prevent-ion and/or disruption of flow channeling is mounted so as to corotate with said rotating mixer head means.

6. An extruder having a hollow barrel, screw means mounted in said barrel to advance material therethrough, mixer head means longitudinally mounted in said barrel near the terminal end of said screw means and gaseous blowing agent inlet means positioned in said barrel near the upstream end of said mixer head means, said mixer head means having in the surface thereof a plurality of alternate land and groove means arranged to extend substantially longitudinally defining a plurality of grooves open at the upstream ends and closed at the downstream ends, said land means having close clearance with the internal walls of said barrel at the point of positioning of said blowing agent inlet means so that said blowing agent inlet means is intermittently opened and closed by said land means to the passage of blowing agent to said plurality of groove means, and land pairs alternately in spaced clearance and close clearance with said walls in the mixing region downstream of said inlet means, said mixer head means having hollow internal passage means positioned therein over a substantial portion of its terminal length, and a plurality of inlet conduits severally communicating between said plurality of land means and said hollow internal passage means, said inlet conduits being positioned between said plurality of alternate spaced clearance and close clear-ance land pairs of said land means which are, in turn, positioned between said grooves.

14.

7. An extruder in accordance with claim 6, wherein means for the prevention and/or disruption of flow channeling are positioned in said hollow internal passage of said mixer head means.

8. An extruder in accordance with claim 6, wherein said means for the prevention and/or disruption of flow channeling is mounted so as to be stationary with respect to said extruder barrel.

9. An extruder in accordance with claim 6, wherein said mixer head means rotates, and said means for the prevention and/or disruption of flow channeling is mounted so as to corotate with said rotating mixer head means.

10. The method of continuously effecting mixing, in an extruder, of a stream of thermoplastic material and a gaseous blowing agent comprising: forming a continuous stream of flowable thermoplastic material; subdividing said stream into a plurality of component streams;
introducing gaseous blowing agent into said plurality of component streams through gaseous blowing agent inlet means, to form mixtures thereof; intermittently opening and closing said inlet means to the passage of gaseous blowing agent; intimately mixing each said component stream;
rejoining said plurality of component streams to form a single stream; and mixing said single stream while prevent-ing and/or disrupting channeling of said stream in its further passage through said extruder.

15.