Note: Descriptions are shown in the official language in which they were submitted.
D-5886
iO~8717
BACKGROUND OF THE INVENTION
4 The present invention relates to a method for
making a thermoplastic or elastomeric product which will
6 cross-link in the presence of moisture on the basis of
7 silane or silane compounds which have been grafted onto
8 the thermoplastic or elastomeric macramolecules.
The German printed patent application 1,963,571
11 discloses a method for cross-linking polyolefine e.g. poly-
12 ethylene or a mixed polymer in the presence of a compound
13 which is capable of producing free radicals in the olefin
14 macromolecules. A silane compound of the formula R R Si Y2
is grafted onto the polyethylene macromolecules, whereby
16 R is a mono-valued olefinically unsaturated hydrocarbon or
17 hydrocarbon oxi-group; Y is an alcoxi-,acyloxi-,oxime- or a
18 substituted amino group; and R is either a group R or a-
19 group Y or methylene. The silane is grafted upon the macro-
molecules of the olefine in an extruder and at a temperature
21 in excess of 140 C. The extruder kneads and masticates the
22 material and mixes its components.After the material has
23 resided in the extruder for a certain period of time, the
24 grafted polymer is extruded, granulated and stored. The grafted
polymer powder may be mixed later with a catalyst batch,
26 extruded as to the final shape, and the resulting product
27 is then placed into hot water to obtain cross-linking.
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1 These known methods are characterized by a
relatively large number of individual steps, such as mixing,
3 grafting/ granulating, etc. Moreover, it was found that the
4 material may to some extent cross-link prematurely, i.e.
prior to obtaining its final shape. This is particularly
6 so, for example, during cooling of the granulate. Thus, the
7 final product is not always satisfactory. For example, the
8 material may be used to jacket an electrical cable,and the
9 jacket requires particular mechanical as well as electrical
0 properties. As to the latter aspect, it is particularly
11 disadvantageous that the known method results in pockets
12 in the material after having been shaped. The insulative
13 properties are, therefore, locally quite weak, so that,for
14 example, the voltage that can be transmitted is limited.
As regards the mechanical properties, the formation of gas
16 pockets may render the surface quite rough,and the jacket,
17 for example, can wear rather rapidly.
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It is an object of the present invention to process a thermo-
plastic of elastomeric material whose macromolecules have been grafted
with a silane compo~ld for later cross-linking in the presence of water.
It is another object of the present invention to process material
that will cross-link, but which requires processing without e.g. premature
cross-linking or other kinds of interference to obtain a higher quality
product.
It is, therefore, a further object of the present invention to
provide for a new method and equipment to make products of thermoplastic
or elastomeric material which will later cross-link after having been shaped
into the final form.
It is a specific object of the present invention to provide a
new and improved method for jacketing electrical conductors.
It is a specific object of the present invention to process a
thermoplastic or elastomeric material to which a graft additive and, possibly,
other compounds have been added.
Accordingly~ one aspect of the invention provides a method of
making a product of a thermoplastic or elastomeric material which is to cross
link subsequent to shaping the material into said product, comprising the
steps of
mixing the thermoplastic OT elastomeric base material with silane
or a silane compound at a ratio to permit grafting of the silane or silane
compound, resulting in a mixture;
mechanically working the resulting mixture at a temperature in -
excess of the melting poin* of the material to homogenize the mixture as
melting at said temperature while obtaining said grafting;
immediately thereafter degassing the still warm, worked, grafted
mixture, still flowing due to the working step; and
immediately after degassing forming the still flowing and now de-
gassed mixture into the said product, the product cross linking subsequentlyin the presence of moisture.
Another aspect of the invention provides an appara~us for making
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a product of a thermoplastic or elastomeric material which is to cross link
subsequently to shaping the material into the product, comprising;
first means for intimately mixing the material with silane or a
silane compound and for mechanically working the resulting mixture, thereby
producing a flow of melted mixture in which the silane is grafted onto the
material;
second means connected to the first means for degassing the melted
mixture of the flow; and
third means connected to the second means for shaping the degassed
mixture into the product in which the grafted material will cross link in the
presence of moisture.
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10~8717
1 In accordance with the preferred embodiment
$ of the inventlon, it is suggested to mix and work, e.g.
masticate and knead or extrude base material (elastomeric
or thermoplastic) and a silane or silane compound of the
type outlined above, to produce a flow of a melted mixture
~ in which the graft reaction occurs, and to degas the
7 material, while still warm and flowing, immediately prior
8 to forming and shaping the mixture into the final form,
9 which it will be made to retain later by cross-linking.
Specifically, the degassing step is to be carried out after
11 completion of grafting. Thus, it is a specific feature
12 of the invention that the material, grafted for subsequent
13 cross-linking, is neither granulated nor stored, but the
14 warm stream of material is degassed up to the point in time
in which it is e.g. form-extruded.
16
17 The inventive method is to be practiced
1~ with specific advantage for insulating e.g. cable, not only
19 low voltage cable (up to one kilovolts), but also for cable
which transmit 10 kilovolts or even more. Even if the cable
21 insulation is to be relatively thick, vigorous degassing
22 will avoid formation of any pockets. The method can also
23 be used e.g. for making plastic tubes or other products.
24
It is of specific advantage to combine the
26 adding of the silane or of a silane compound to the base
27 material with the adding of additional substances needed and/
28 or desirable for grafting and/or for the final product.
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1 The silane plus additives is a solution which may either
2 be added to the elastomeric or thermoplastic base material
3 as it is worked and melts, or the said solution may be
4 used to wet the powdery base material, as it is charged
S into the working device. In either case, mixing of the
6 plastic with the silane and homogenizing the mixture by
7 working it through, i.e. by kneading, extruding and/or
- 8 masticating may be combined so that a homogenic mixture
9 of the gradually melting plastic with all the other additives
0 and the silane i~s obtained in a single step, and a continuation
~, ~ ~k\e ~
11 of the rr~atment- of the material at rising temperature will
12 result in the grafting of the silane onto the macromolecules
13 of the base material at such temperature.
14
By way of example, a thermoplastic material,
16 such as polyethylene has to be blended with a silane compound
17 for grafting. The grafting, however, requires the adding
18 of e.g. peroxide for graft initiation. The peroxide, e.g.
19 an esterperoxide, will decay later thermally and furnishes
the radicals needed to graft the silane onto the polyethylene
21 macromolecules. A typical example here is e.g. dicumyl
22 peroxide or, preferably, an ester peroxide having a low cross-
23 linking effectiveness, because the principal function of
~4 the peroxide is to obtain the silane grafting. For example,
one may use here tert.-butylperoxi-isonanoate with 1,3 bis
26 (tert.-butylperoxi-isopropyl) benzene. This graft initiator
27 is to be added at about 0.05 to 0.5 parts by weight
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1 per lOO parts base plastic material.
3 Additionally, an activator will be added,e.g.
4 a polyfunctional monomer, such as triallylcyanurate,
divinyl benzene, ethylene dimethycrylate or triallylphos-
6 phite. One needs here about O.Ol to lO parts by weight per
q lOO parts base polymer. Another additive is an anti-oxidant,
8 preferably being a substance that can also be grafted onto
9 the macromolecules to remain stably in the final product so as
to provide permanently protection against oxidation. One may
11 use here an oligomer or monomer of 2, 2, 4 Trimethyl-dihydro-
12 quinoline at about 0.05 to 2 parts, preferably less than
13 0.5 parts by weight per lOO parts of the base polymer.
14 Specific anti-oxidants to be used in this capacity with
advantage are, for example, the following. Certain derivatives
16 of the 2, 2, 4 trimethyl-dihydroquinolines are tradcd under
~ ro~e -~n ~ r~ts
17 the ~eYi~}~# Anox HB, Flectol H and Agerite resin D.
18 Other anti-oxidants include specifically the monomer quino-
19 line derivatives, such as 6-ethoxy-2,2,4-trimethyl dihydro- ~ -
quinoline (also called Santoflex~AW) and the 6-dodecyl 2,2,4-
21 trimethyl dihydroquninoline (also called Santoflex~DD). The
22 last two substances have the specific advantage that they
23 are liquidous at room temperature and can be particularly
24 easily and homogenously distributed in the base polymer.
26 Still other additives are those which will
27 develop water internally in the material for obtaining the
28 cross-linking.-Also, a condensation catalyst, e.g. dibutyltin-
29 dilaurate or a heavy metal salt or a long fatty acid can be
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1 added. However, for reasons of avoiding premature cross-
2 linking it may be advisable to add the catalyst after
3 degassing, just as the material is made to assume its
4 final shape. This is particularly so if the catalyst is
provided for accellerating a silanol condensation reaction.
7 As stated, these various components (plastic,
8 silane, additives) are intimately mixed, homogenized and
9 heated so that the plastic melts and a homogenized molten
mixture results. The initial phase or operation may include
11 separately metering quantities of plastic powder and of the
12 silane with additives, and these metered quantities are
13 combined (added together) in the entrance of the mixing device
14 and processed together thereafter under continuous homogeni-
zation and gradual heating and melting.
16
17 Generally speaking and in the preferred form
18 of practicing the invention, the various components should
19 be mixed and worked within a very small spectrum of their
individual dwell and residence times in the equipment so
21 that all parts of all components are uniformly and homo-
22 genically distributed throughout the mixture. Working, moreover,
23 should be carried out gently under avoidance of peak tempera-
24 tures, even temporary ones, particularly to avoid any premature
cross-linking.
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1 Alternatively, the silane with additives may be
added in steps to the worked, heated and melted plastic
3 to obtain a gradual buildup of the silane and additive
4 content. In either case, the molten mixture is homogenized
and the silane is grafted onto the macromolecules of the
6 base material when the necessary temperature has been reached.
7 The working temperature should be in excess of 160 C, but
8 not higher than about 250 C, except that a temperature of
9 up to 270 may temporarily be permitted; preferably one will
0 work in the range from 180 to 230 C. The graft initiation
decays at these temperatures so that grafting is obtained.
12 indeed.
13
14 The flowing, viscous, grafted material is amenable
to develop gas if that gas is permitted to remain. Pockets
16 may be generated by excess silane or otherwise in the final
17 product. Thus, the gas causing and filling these pockets has
18 to be removed before the pockets can be formed and set. This
19 is the reason for degassing the warm flowing mixture, which
has been worked through, homogenized and grafted. Thus, the
21 flow of grafted material is subjected to a low pressure, such
22 as 1 to 200 Torr, preferably, however, 20 to 50 Torr, while
23 still being warm, having a temperature of 130 to 240 C,
2~ preferably about 200 C.
26 The material flows directly from the low pressure
27 zone of degassing to the form-shasping device, such as an
28 extruder head. The degassing involves, as stated, the removal
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10~87~7
1 of excess silane; hut also volatile peroxide reaction
product tfollowing the grafting) air and other impurities.
3 The resulting product is free from gas and otherinclusions
4 that could form pockets. The resulting product has, there-
fore, more uniform properties and is very suited for high
6 voltage insulation. The work shaping tool is, for example,
7 an extruder and the low pressure zone for degassing may
8 be provided for in the entrance or feeder section of the
9 extruder, from which an extrusion screw moves the degassed
material to the extruder head.
11 ' '
12 - The feeder section of this extruder is
13 preferably connected to the outlet of a kneading and
14 masticating machine, which is charged with the base material
and into which is fed the silane solution, for example,
16 in steps to to stepwise enrich the content of base material
~7 with silane plus additives. Since the amount of liquid
18 to be added (silane solution plus additives) is relatively
19 small, graduated stepwise feeding in a continuous process
of melting the plastic is quite advantageous, regarding
21 homogenization. In this case, at least some of the silane
22 solution is fed into the already melted - worked plastic.
23
24 Alternatively, the mixing and working
device may be a second extruder being charged through
26 metering devices with the various components. This second
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108~71'7
1 extruder serves as kneading and masticating machine and
has along its screw a mixing zone and a grafting zone at
3 elevated temperatures. The extruders may form an L-shaped
4 configuration.
6 Grafting occurs in either case in the portion
7 of the homogenizing machine or device close to the outlet
8 as connected and leading to the degassing zone~and the kneading
9 device gradually increases the temperature of the mass to
180 to 250 C, possibly even to 270 C. Degassing is then
11 carried out at the temperature the flowing melt has as
12 emerging from the kneading and masticating device.
13
14 In order to enhance degassing, it may be
advisable to cause the warm material to flow in several
16 streams so as to increase the surface / volume ratio.
17 Following degassing the material should move promptly and
18 completely to the final work shaping tool. Any stagnation
19 should be avoided, and material not moving in the desired
direction towards the final shaping tool should be caused
21 to move out of the equipment. Stagnating material may
22 become crusty and even char, and flakes may then be mixed
23 with good material thus ruining the product. In the case
24 of an extruder, the rear portion of the screw may have
reverse flights.
26
27 As far as equipment is concerned, the pre-
28 ferred`form of practicing the invention involves a combined -
29 mixing and working (masticating) tool, which feeds into
the degassing chamber, which, in turn, is the feeder
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1 inlet or the like of the work shaping tool. As to the latter
2 combination, immediacy of shaping following the degassing
3 is of the essence. As to the preliminaries, however, it
4 should be mentioned that mixing, melting and grafting may
be carried out in separate devices but on a continuous basis
6 as far as melting and grafting-during-working is concerned.
8It may be advisable to control the rate of
9 feeding mixed and grafted material in dependence upon the
pressure in the degassing zone as too high a feeder rate may
11 result, initially, in too strong a gas development so that
12 the pressure increases too much which in turn will retard
13 and impede the degassing. Therefore, it is advisable to
14 provide a feed-back loop which retards the mass flow if the
pressure in the degassing zone increases too much.
16
17The base material is preferably provided
18 in powdery and granulated form. The silane, preferably with
19 the additives in solution is mixed with the powder, e.g. in
a funnel-shaped hopper feeding a mixing extruder (which is
21 the mixing and masticating machine). The powder may be metered
22 through a conveyor belt weigher and the liquid may be metered
23 through a pump; both devices may operate continuously or
24 intermittently to obtain metered batches of fixed ratios of
the several components, which are then gradually fed to the
26 kneading device. In either case, the powder is wetted with
27 the silane solution~and the residence time of all parts is the
28 same throughout the subsequent homogenization, melting and
29 grafting.
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1 Additionally or alternatively, the powdery base
$ material may be mixed with the silane and additives prior to
3 working, using e.g. an agitator operating at 500 to 3000 RPM,
4 so that prior to melting and working the liquidous components
diffuse into the powder particles. Continued agitation raises
6 the temperature to enhance diffusion and to maintain the wet
~ powder in a fluidized state with uniform distribution of
8 wetness to obtain uniform diffusion. The material may then
be melted and masticated to obtain the grafting which is then
followed by degassing and shaping.
11
12 As stated, the mixer may be charged intermittent-
13 ly or on a continuous basis and may be included in a funnel
14 or hopper that feeds, e.g. the melting-grafting extruder.
The base material when in a granulated or powdery state will
16 be wetted by the added silanè plus additives solution and
17 mixing occurs already in the hopper. The amount of liquid
18 added is relatively small, so that the liquid should wet all
lg of the grainy or powdery particles. Intimate mixing is, there-
fore, quite important.
21
22 It should be noted that upon feeding the
23 plastic powder and the silane solution concurrently into the
24 receiving hopper of the kneading machine or device one obtains
2~ an interaction between just added material and wetted granular
26 material further down in the funnel or hopper. The latter --
2~ material produces some evaporation of the liquidous portion,
28 but the vapor condensates on the just added powder, thereby
29 cooling the latter. Also, this way not portion of the
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1088717
1 added material (silane plus graft producing and enhancing
$ additives and others) will be lost. This phenomenon occurs
3 also if there is no agitation in the funnel, but if mixing
4 is carried out only in the first portion of the masticating
tool.
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2 DESCRIPTION OF THE DRAWINGS
While the specification concludes with claims
6 particularly pointing out and distinctly claiming the
q subject matter, which is regarded as the invention, it is
8 believed that the invention, the objects and features of
9 the invention and further objects, features and advantages
thereof will be better understood from the following des-
11 cription taken in connection with the accompanying drawings,
12 in which:
13
14 Figure 1 is a schematic view of equipment in
accordance with a first example for practicing the invention
16 in accordance with the preferred embodiment;
17
18 Figure 2 is a schematic view of equipment
19 representing the best mode for practicing the preferred
embodiment of the invention; and
21
22 Figure 3 shows a modification of the device
23 of Figure 2.
24
Proceeding now to the detailed description of
26 the drawing, Fig. 1 illustrates a mixing, kneading and
27 masticating machine 1. This machine has an entrance chamber
28 ~ S
13~driven by a motor 12 via a suitable transmission and a
29 speed reducing gear 11.
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~o~ 13
~ is or includes a hopper and, possibly, an agitator
2 to retain the powder in a fluidized state. The hopper receives
~ the granulated or powdery base material, e.g. polyethylene
4 from a source for such material. The various additives, such
as silane or a silane compound~ a peroxide, an activator,
6 possibly, but not necessarily a condensation catalyst and,
7 possibly, an anti-oxidant have been previously blended into
8 a solution and are fed to the chamber via ducts and inlets 2.
9 Some of the additives may already be added to the base material
prior to entry in kneading machine 1.
11
12 A mixing and kneading machine very suitable for
13 this purpose is, for example, a so-called "Buss-Kno-Kneter"
14 which is used generally for working and homogenizing granu-
lated material. This machine includes a mixing and working
16 chamber containing a threaded worm or screw which rotates
17 and undergoes also a reciprocating movement. The worm proper
18 has a shaft with wing elements cooperating with stationary
19 teeth which project from the chamber wall, of course, on the
inside thereof. The feeder ducts 2 for the additives preferably
21 run into the fixed teeth inside of chamber 1, which is the
22 reason for the staggered arrangement.
23
24 Rotation and reciprocation of the worm results
in a coaction with the stationary teeth which kneads and
26 masticates the material fed thereto and mixes intimately its
27 components. The working, possibly supplemented by auxiliary
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1()~8'717
1 heating, raises the temperature to between 160 to 250 C,
2 preferably between 180 and 230 C. The silane solution is
3 added in steps through the various ducts and is thereby
4 blended with the melted plastic in relatively small quanti-
ties in each instance of feeding, so that the silane and
6 additive content in the plastic is enriched in steps. There-
q fore, the silane solution is added to the plastic to a
8 considerable extent when the plastic is already in the melted
9 state. The result is a homogenic mixture which is obtained
rather speedily. Moreover, the peroxide decays thermally
11 and gives off radicals which cause grafting of the silane
12 and of the oxidant onto the macromolecules of the plastic.
13
14 If the material is worked at the range of
about 180 C, it will have a temperature when emerging from
16 chamber 1 through outlet 3 of about 200 C to 210 C. The
lq material thus heated and homogenously mixed ~grafting
18 having been at least substantially completed) is fed to a
19 degassing chamber 4 being the entrance chamber of an extruder 5.
Degassing results from maintaining a rather low pressure in
21 that chamber, preferably below 100 Torr. The preferred opera-
22 ting range is about 20 to 50 Torr. As a consequence, all
23 trapped gas, particularly excess (i.e. not grafted) silane
2~ is, in effect, pumped out by-operation of a pump 14, which
may be connected to the extruder 5 in lieu of the usual
26 hopper. A rather wide range of temperature is permissible
27 here, such as from 130 to ~40 C. However, about 200 C
28 are preferred for degassing.
29
The feed screw extruder 5 is driven also :
31 here by a motor 15 via a shaft and a speed reducer gearing 16.
32 Extruder 5 serves as the tool for shaping the material in
33 final shape prior to cross-linking.
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By way of example, the extruder may be constructed so that an
electric conductor 7 passes through the extruder head 6, and a
jacke~ or envelope is extruded about that conductor. The
extruded product is free from pockets and trapped gaseous im-
purities. The jacket will have a very smooth surface as trapped
gas bubbles, which could pop right on the surface, have not
formed. Also, since internal gas-filled pockets are substan-
tially absent, the mechanical and electrical properties are
- about as predicted on the basis of impurity-free material.
The jacketed conductor may then be stored, e.g.
in water to obtain cross-linking. However, additives to the
base material may have included a compound which yields water.
Examples are disclosed in United States letters Patents
4,048,129 see also Canadian Application 23~826 of common assignee.
It was found that the following material is best
suited for a cable jacket: ;
` Polyethylene (MJ 8) 100 parts by weight
2,2,4 Trimethyl-dihydroquinoline 0.5 " "
Tert. butylperoxi-isonan~ate 0.2 " "
1,3 Bis (tert. butylperoxi- 0.02 " "
isopropyl) benzene
Vinyl-trimethoxysilane 2.0 " "
Triallylcyanurate 0.1 " ~' -
Dibutyltindilaurate O.OS " "
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1 Proceeding now to the description of Figure 2,
there is shown a metering device 21, by means of which base
3 material e.g. an olefinepolymerisate or a mixed polymerisate
4 of olefines is dispensed at a metered rate and into a funnel
or hopper 22. The preferred material used here is also
6 polyethylene which may be dispensed onto the metering device
7 21 in granulated consistency.
9 The hopper 22 is fed additionally with additives
by means of a pumpYwhich feeds silane in liquid form to the
11 funnel or hopper 22. The rate of feeding is controlled so that
12 it is commensurate with the rate of feeding of powdery base
3 material. One will use about 0.5 to lO parts of silane per
14 lOO parts (by weight) of base polymer, e.g. polyethylene.
The silane solution will thereby wet the powder particles
16 as the silane drips into the powder. Some of the silane will
17 evaporate, particularly as the wetted powder drops deeper
18 into the funnel. However, the silane vapor will recondensate
19 on the powder particles that follow from above, so that these
particles serve as a kind of feedback coolant for the silane
21 that continues to drip onto these following particles.
22
23 Concerning the additives, they should include
24 a free radical initiator, such as a peroxide at an amount
of 0.05 to O.5 parts by weight per lOO parts by weight base
26 polymer ~see example above). Another additive needed (and
27 to be mixed with the silane) is an activator and here one
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1(~81~'717
may use a polyfunctional monomer of the type mentioned
above.
3 Another additive to be included in the
4 silane solution is an anti-oxidant and here particularly
one that can also be grafted onto the macromolecules of
6 the base material. One should use here an oligomer or the
7 monomer of the 2, 2, 4 trimethyl dihydroquinoline. Such
8 compounds include a rather reactive C-C double bond
9 which permits radical grafting on the hydrocarbon chain of
the polymer of the base material. This way, the stabilizing
molecule (anti-oxidant) will be fixed to the macromolecules
12 of the highly polymerized granulate or powder and thus
3 cannot migrate out of the material or volatilize; it will
14 be permanently present in the final product and provides
lasting protection against oxidation.
17 As stated, all those additives (except,
18 preferably, a condensation catalyst and water producing
lg substances) are included in the silane solution, which is
dispensed onto the polyethylene powder in funnel or hopper 22
21 by means of pump 24. A casing 25 encloses the equipment
22 21 and 24tand funnel 22 is the exit of casing 25. The casing
23 should seal these parts gas-tight as no substance should be
24 permitted to evaporate and escape.
26 Funnel or hopper 22 is also the inlet to a
27 first extruder 23. The funnel or hopper 22 may-include a stirrer
28 to provide for continuous agitation of the wet powder. One may
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use here a grooved sleeve as part of the funnel 22 to make sure
that the wet granulate or powder 26 advances properly into the
extruder 23. The extruder 23 has a barrel or cylinder whose
diameter D is about 150 mm and which is 25 x D long. As a rule,
this extruder is at least 20 x D long but may have a length of
up to 30 D.
The powder or granulate 26 wetted with silane solution
is now intensively mixed by means of the rotating feed screw 27
of extruder 23. The material is additionally heated and melts,
so that the flowing material becomes very homogenically mixed.
Melting occurs rather gently. Mixing and melting occurs in the
first range of the extruder (as seen from the inlet funnel 22)
and having length of about 6 D to 10 D. The temperature here is
about 120 to 170 C. The second extruder range, downstream
of the first one and being about 24 to 10 D long is provided for
grafting. At a distance about 8 D from the hopper 22 and further
the temperature has risen to at least 180C and may continue to
rise up to 270C.
; The extruder 23 is laterally connected to a second
extruder to establish an L-shaped two-extruder configuration.
The connection is denoted by numeral 28 and constitutes an out-
let nozzle of extruder 23. Actually, this nozzle may be com-
prised of multiple openings for extruding plural streams of
flowing mixture to increase the surface of that material in
relation to the volume. The connection is made, e.g. by flang-
ing the first extruder to the second extruder 29 at a point
being about 3 to 4 D in the extruder cylinder or barrel of the
second extruder 29. This second extruder may have also a
diameter of 150 mm but its length is somewhat shorter, namely
about 15 D.
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1 The,entrance portion of extruder 29, i.e.
$ that zone into which extruder 23 feeds, is established as
3 degassing chamber 32. That chamber 32 is connected to a
4 pump 35, which maintains a pressure of 1 to 200 Torr. The
pump discharges the gas that emerges from the material.
6 A pressure transducer 36 is provided to monitor the pressure
7 in chamber 32 and controls the feeding of extruded, grafted
8 polymer into the degassing zone. The purpose thereof is
9 that if too much material is fed by extruder 23 the pressure
0 in chamber 32 will rise (e.g. at top rate of operation,of
11 the pump 35) and degassing will become insufficient. Thus,
12 the feeding should be retarded. In view of the inherent
13 delay between changes in the rate of feeding extruder 3
4 and the effective change in flow at outlet 8, the response
pressure for throttling the feeding should be comfortably
6 below permissible maximum.
17
18 The material is additionally homogenized
19 in extruder 29 as the feed screw 33 thereof advances the
material to the extruder head 30, being also constructed
21 for passage of a conductor 31, about which a jacket is being
22 extruded. At 37 a catalyst and water releasing or producing
23 substances may be added. The extruder 29, therefore, serves
24 additionally as homogenizing and mixing machine as to these
additives, while the temperature remains or is kept rather
26 high (about 200 C or even higher). The catalyst to be added
27 is a hydrolysis or condensation catalyst, such as dibutyltin-
28 dilaurate or a heavy metal salt or a long chain fatty acid.
29 It is safer to add the condensator catalyst after the
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degassing of the silane grafting polymer, because this
2 particular additive is not needed to obtain grafting of
3 the silane. About 0.5 to 10 parts by weight catalyst
4 per 100 parts of base material should be used.
6 The other additive which may be charged during
q form-extruding is of the type which forms water by chemical
8 reaction, for example, stearic acid or adipin acid together
g with a non-hygroscopic material, such as tin oxide or zinc
o oxide. As these materials react, water and an indifferent
reaction product (e.g. a metal salt) is produced. However,
12 the reaction time of water formation is rather slow, so that
even if these additives are included already in the base mater-
ial or even in the silane solution, very little premature
cross-linking will occur. The reaction will occur primarily
16 after the composite material has been form-extruded and the
17 water as developed will cause the cross-linking. This additive
1~ is added at a proportion of 0.05 to 5 parts by weight per
19 hundred parts base polymer. Irrespective of the inclusion
Of a water releasing additive, the extruded jacket is caused
21 and/or permitted to cross-link subsequently to leaving the
22 extruder head which amounts to a stabilizing step as to the
23 shape this product has obtained during this second extrusion.
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1 It can thus be seen that the equipment pro-
2 vides for a direct sequence of these operational steps,
3 including mixing-homogenization, grafting, degassing, and
4 form-extruding, whereby also here a degassing step separates
the yrafting from extruding, but the latter follows imme-
6 diately the degassing. As a consequence of vigorous degassing
q as preceded by extensive homogenization of the blend, the
8 insulation jacket may well have thickness not much, if at
g all, above the theoretical minimum for the particular material.
11 In order to avoid accumulation of material at
12 the back of the worm, which could crust or even burn, flake
13 off and mix with regular material, the extruder 29 is con-
14 structed to remove such material. Accordingly, the rear
portion of the worm has threads which are reversed
16 flights, i.e. the screw threads have opposite pitch. Thus,
lq any material not advanced towards the extruder head lO is
18 advanced in the opposite direction and removed.
lg .
Figure 3 shows that the feeding of extruder
21 23 can be provided for in a somewhat modified version. Both,
22 the weighting conveyor 21 and the feed pump 24 work intermit-
23 tently and feed fixed quantities into a mixer 40, which
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1 homogenizes the powder-silane solution to obtain a homo-
2 genically wet powder amounting in effect to a coating of
3 the polyethylene particles with silane solution. The
content of this mixer 40 is then discharged into the hopper
of extruder 23, from which it is gradually drawn into the
6 extruder 23. In the meantime, a new batch is prepared
7 through metering the proper amounts of powder and silane
8 solution, mixed in the mixer and held in readiness for
9 replenishing the hopper content.
1 The invention is not limited to the embodi-
12 ments described above but all changes and modifications
13 thereof not constituting departures from the spirit and
14 scope of the invention are intended to be included.
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