Note: Descriptions are shown in the official language in which they were submitted.
~` D--5700
BACYGROUND OF T~:E INVENTION ` `
7 The present invention relates to the graftiny
8 of polymers for purposes of obtaining cross-linked thermo-
9 plastics ox elastomers, and more paxticularly the invention
relates to grafting a silane compound upon the not yet
11 cross-linked molecules o~ a polymer to obtain later cross-
12 lin~ing in ~he presence of moisture. Thus, the invention
13 relates to the prepaxation of plastic or elastomeric
14 material~or subse~uently obtaining cross-linking, the
preparation being the yra~ting of silane upon the polymer.
16 The plastic material so treaked is to include olefinepoly-
17 merizates or olefine-mixed pol~nerizates and others,
A ~
tt 18 pre~erably polyethylene, and to be~ as jacket, envelope
~9 or cover for elongated material such as electrical cable,
20 co~ductors or tubes, etc. `
21
22 The German printed patent application DAS 1, 79
23 028 discloses gra~ting alkoxy-silane componenks on oxganic ~-
24 polymerizates wi~h subsequent ~uring in the presence of
.. . .
moisture to obtain cross-linking. An alkoxy-silane co~
;~ 26 pound may have the structure R Si Y3 wherein R is a vinyl
- 27 group or a gammamethacr~lperoxipropyl group, and Y is an
28 alkoxy group with less than six carbon atomsO
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Particularly, this printed patent application discloses grafting
of trimethoxy-vinyl-silane or triethoxy vinyl-silane or other functionally
organic silanes on polyethylene macromolecule. The grafting of the silane
compound is specifically obtained pursuant to extrusion in the presence of
additives, which provide for suitable radicals such as peroxides, whereby
the grafting requires rather high temperatures, such as from 180~ to 220C.
Prior to ex~rusion, silane and peroxide are mixed with granulated poly-
ethylene. In fact, the polyethylene particles are surface coated with the
liquidous mixture of silane and peroxide. Homogenization is produced later
in the extruder, which also perorms the grafting. The resulting product
is granulated again, and the grafted, granulatsd copolymer is blended with
polyethylene in a polyethylene batch which contains also a catalyst for
cross-linking. The resulting final product is then form extruded e.g. it
is extruded as a jacket around a cable, a tube or the like.
This known method has the disadvantage that the usual extruders
produce local premature, cross-linking due to non-homogeneous peroxide
distribution. The formation of such gel particles is particularly dis-
advantageous when the resulting product is to serve as electrical insulation.
Thus, grafting will be carried out properly only if the temperature profile
in the barrel of the extrusion process is adequately predetermined and
controlled so that grafting proper occurs only after the materials have been
homogenized sufficiently.
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1 As stated, the coatin~ of the granules intro-
2 duces some inherent inhomogeneities in the distribution
3 of peroxide, and premature cross-linking during cooling
of the granulated graEtiny component i5 detrimental to
this process. Therefore, cable insul~tion cannot readily
6 ba made in that manner particularly in those cases where
7 electrical and mechanical pxoperties are critical. The
8 outline of the process above indicates further that a
9 multiple of separate steps have to be performed which
complicates the process. Also, the grafked but not yet
11 cross-linked plastic can be stored only to a limited
12 extent and or a limited duration as any moisture will
13 start the cross-linking. Ik is apparent, however, that
l~ cross-linking should hegln only after the plastic has
assumed and has been given its final shape.
16
17 Another disadvantage of the known procedure is
18 to be seen in that a-fter the catalyst has been added ~the
19 grafting component was added earlier~ ex~rusion becomes
more complicated as long as even traces of moist~re are
21 present in the material~ ~s a conse~uence, the surface
22 of the product is quite rough which is a signi-ficant
23 deficiency if the product is subjected to an electric
24 field and voltage~
26
27
28
29
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6 DESCRIPTION OF T~E INVENTION
`- 6
q It is an object of the present invention -to
8 provide a new and improved method o grating a silane
9 compound upon thermoplastic or elastomeric material to
:
-; 10 permit such material to cross~link in the presence v~
ll moisture while avoiding the problems and interferences
12 outlined above.
13
l~ In accordance with the preferred embodiment of
the present invention, it is suggested to provide the
16 thermoplastic or elastomer as dry, fluid powder which is
.,
17 agitated for internal, rapid, high speed, turbulent move-
18 ment. Silane or silane compounds and other additives, all
l9 in liquid form, are added to that powder before and/or
during the fluidization while subjecting the powdex to a
21 higher-than-normal temperature, but below the crystallite
22 melting range ~e.g. in the range from 60 to 100C) so
23 that the additives diffuse at least partially into the
:.......................................................................... .
24 suxface o~ the ~luid-particles to be distributed therein
as homogeneously as possible. The thus treated material
26 does not loose its character as a fluidizable pbwder. That
27 powder is subsequently subjected to steps by means o~ which
28 grafting is obtained.- It should be mentioned, that the
29 term powder is used here in the general sense and is to
include also grit like material and material of granulated
31 consistencyO
32
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~ ,
The present invention provides in a method for cross-linking
thermoplastic or elastomeric material, wllerein lOO par~s by weight of
the material are mixed with 0.5 to 10 parts by weight sllane under
conditions o obtaining a homogenic mixture and wherein the silane is
grafted upon the molecules of the material, the improvement of adding
water-containing additives to the mixture, the water being released at
elevated temperatu~e to cause cross-linking of the silane grafted material
in the presence of moisture.
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It can thus be seen, that prior to ~he final shaping
extrusion, separate extrusion for grafting and subsequent ~ranulation
is not necessary; the material never looses its powdery, 1uidizable
consistency. This is clearly simyler and more practicable.
The additives themselves are homogenized prior to the
mixing with the fluidized or fluidizable powder. These additives
are the silane and/or silane compounds as used for grafting; free
radical initiators such as peroxides; polyfunctional monomers as
activators such as e.g. triallyl cyanurate, divinyl benzene, ethylene
dimethycrylate , triallylphosphite and others; condensation catalysts
such as dibutyltindilaurate or heavy metal salts of long chain fatty
acids may be included at that point or added later. These materials
are mixed, at least in parts, prior to being added to the powdery
thermoplastic or elastomcric material. The latter powder should be
predried to prevent premature cross-linking, so as to improve the
quality of the product. The thermoplastic or elastomeric material ~;
could be tpreferably) polyethylene or a copolymerizate of ethylene with
another co-mo~omer such as ethylenevinylacetate; propylene; butylene,
etc.
The grafting proper can be obtained in various ways. By
way of example, the silane or silane compound can be grafted on the
macromolecules o~ the polyethylene under mechanical-thermal treatment
e.g. while being form-shaped in an extruderr The treatment requires
temperatures between 160 and 250C; that range depends essentially on
- 6 -
~57~
1 the decomposition kineticsof the particular additives that
2 is to furnish the active radicals. ~lso, dwell time in
3 and throughput of the extruder are factors which determine
¢ the operating temperature.
6 Alternatively, grafting may be obtained by sub-
7 jecting the powder with diffused additives to high energy
8 radiation. Peroxide is not needed here as an additive
g and the quality of the resulting pxoduct is even higher.
The melting temperature may be less than usual, e.g. below
ll lgOQC so that the entire thermal operating range to which
12 the material is subjected is lower. By way of e~ample,
13 one may u3e here a Van-de-Graaf accelerator emitting elec-
~ trons a~ a doses o~ 0.1 to 3 r~d~ a~ter having extruded
the material into the desired shape. This was particularly
16 used for Example II (infra).
17
18 It may be advisable to include among the additives an
19 anti~oxidant for thermal stabilization. This is particularly
advantageous if the anti-oxidant is of the variety which
21 influ~nce~s ver~ little the reaction involving the radicals.
22 Rather, they are included in the grafting and are being
23 grafted on the polymer macromolecules.
24
In the preferred form of practicing ~he invention
26 the condensation catalyst will be added af~er the silane
27 plus additives have dif~used into tha particles of the
28 fluidizable polymer. By way of example, a batch is pre-
29 pared of the same granulated powdery pol~ner and mixed with
the condensation catalyst. That batch is then added to the
31
32
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-7-
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l fluidized polymer in the last phase of agitation or in the
2 initial, char~e phase of subsequent extrusion,, Alterna-
3 tively, the catalyst can be sprayed into the extruded
substance or onto the extruded product.
6 It i5 another feature of the invention to use an
7 additive which releases water e.g, at high temperature so
~ that the cross-linking in the presence of moisture results
9 from the internal development of water.
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19
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~ DESCRIPTION OF THE DR~WINGS
~ While the specification concludes with claims
8 particularly pointing out and distinctly claiming the
9 subject matter which is regarded as the invention, it is
10 believed that the invention, the objects and features of -
11 the invention and further objects, features and advantages
12 thereof will be better understood from the following des-
13 cription taken in connection with the accompanying drawings
14 in which:
16 The Figure shows schematically the carrying out `
1~ of the process in accordance with the preferred embodiment.
18
19 ReferenCe numeral 1 denotes a high spe~ed mixer
being, for example, a vessel in which cutters, blades,
21 vanes or the like rotate at high speed, such as 500 to
22 3000 ~P~. Polyethylene if used as starting material is
23 charged to that mixer~ Reference nu~eral 10 denotes a
24 grinding and predrying stage so that the material when
charged isin a fluidizable state. Thus, the polyethylene
26 as used is in a powdery or granular state. The mixer
27 vessel itself is evacuated so that in fact residual mois- -
28 ture is removed. The evacuation may in fact suffice for
2~ drying.
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32
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The mixing speed is selected in accordance with the mesh
size of the particles. While nothing prec~hudes the application of
external heat~ it was found to be sufficient to run the high speed
agitator at such an intensity that particle collision and friction
actually suffices to heat the fluidized powder and granulated polymers.
Since the surface of the individual particles may begin to soften,
agitation is still continued, though possibly at a reduced rate, but
sufficient to prevent agglomeration caking and clustering of
particles.
The purpose of softening and melting the particle surface
is to enhance the later ensuing diffusion and to increase the diffusion
rate. Since the dif~usion is the principle purpose of agitation,
raising the temperature is quite important. High speed agitation
may not suice so that additional heating may be needed. The agitation
of course will be hi.ghly instrumental in rapidly equalizing the temper-
ature throughout the fluid material so that thermal conduction occurs
only from particle to particle and inside of each particle, but the
mixer~induced turbulence equalize~ the temperabure in and throughout
the vessel.
As high speed agitation and 1uidization takes place the
needed additives e.g. silane and peroxide and other additives are
fed to the mixer These additives are in the liquid state, but due
to the evacuation a considerable portion will evaporate immediately
so that only little liquid remains. The temperature o the material
may have been raised to 80 to 100C, the surface of the particles has
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~ .. . . - . - ~ :, .
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1 softened so that the additives diffuse readily into the
2 powder or granulate particles. The diffusion involves
3 both, gaseous and liquid state of the additives, and the
4 high speed agitation which continues~makes sure that the
particles continue to remain sepaxate and individual ele-
6 ments of the fluid and do not agglomerize wl~ile the
7 additives can readily diffuse into the particles.
9 Friction and collision, of course, continue so
that thermal action is maintained and the diffusion will
11 be completed within a few minutes. ~oreover, the additives
12 become rather homogenicall~ distxibuted, not only thxoughout
13 the fluiclized rnateria] as such, but also within each particle.
14 The pxocess,thus,is continued ~or a period which is kno~rn in
advance, namely the time it takes fox the additives to diffuse
16 deeply into the particles and is more or less homogenically
17 distributed thexein. ~Iowever, the agitation speed may have
1~ been xeduced as it takes more mixing powex and agitation to
19 xaise the tempexature than to keep the tempexature constant
ox approximately constant for diffusion. The continued
21 agitation is needed to prevent the rather warm paxticles
22 from clustering. Also, the yaseous phase should paxticipate
23 in the diffusion so that the maintaining of the fluidized
2~ bed is desixable throughout. Unli~ce the known methods, the
paxticles are not just coated but impregnated thoroughly by
26 the additiveS.
27
28 As outlined eaxliex, the additives which axe ¦~
29 caused to diffuse by the agitation process, have been
mixed with and/or solved in the liquidous silane or silane
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1~'7~
1 compound which acts, therefore, as solvent for peroxide,
2 activators, and anti-oxidant. The catalyst could be added
3 to that solution, but its addiny to the mixture is pra-
4 ferabl~ deferred.
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12
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22
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These additives will therefore also diffuse into the particles of
the fluidized polyethylene powder and the resulting homogeneity in
the distribution will depend on the homogeneity of the mixt~re, bearing
in mind that the agitation will also homogenize t:hese additives even
before diffusion begins.
The solution of additives could be added to the powder
even prior to high speed agitation, even when the fluidized poly-
ethylene is still cold. However, in order to obtain a rapid onset
of diffusion, a high diffusion temperature and a large (înitial)
diffusion gradient should concur, so that ~he adding of the additives
may be deferred until heating and/or agitation has rai~ed the temperature
to about 80C. However, i~ will be appreciated that adding-while-
agitating under vacuum is more complicated than charging the mixing
vessel with the additives prior to agitation.
If one agitates at high speed within the stated range and
causes diffusion at temperatures between 80 and 100C, that process
will be completed in 3 to 20 minutes. Since diffusion is an e-function
process, little is gained by much longer agitation. Also, the finer
the powder, the more rapidly does one obtain homogenic diffusion in
the powdery polymer. The condensation ca~alyst could be added at
that point, but that aspect shall be described later.
- 12 _
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~)S7~8
1 Upon completion of the agitation, an extruder 2
2 is charged with the content of the mixing vessel. The
3 extruder 2 is of conventional variety and of the type used
4 to extrude granular su~stances to jacket a cable or a tube.
The cable or tube 3 to be jacketed thus runs throuyh the
6 extruder and a jacket is extruded around it. The extruder
q is operated at a temperature of the extruded material to
8 be in the range ~rom 160 to 250C, p~eferably from 190
9 to 230C. That extruder does not have to perform homo-
genization work. Therefore, one does not need special
11 worms or screws which perform in parts mixing of th~ raw
12 charge. The extruder (barrel) chamber is however, the
13 location in which grafting occurs, just prior to jacketing
14 the cabl~ or tube.
'15
16 The jacketed cable or tube 4 as it leaves the
17 extruder passes, for example, through a spray device 5 by
18 means of which the catalyst (for cross-linking) is applied.
19 Thus, in this form of practicing the invention the catalyst
is not part of the additives. The catalyst is therefore in
21 contact with the silane for a minimum period of time, just
22 ahead of the cross-linking. This way, the speed of cross-
~3 linking once initiated by application of moisture is rather
24 high.
26 The cable is subsequently wound on a drum 6 which
27 may be finally placed into a tank of water, 7, and stays
28 therein or a period sufficient for the water to diffuse
2~ into the envelope so as to obtain cross-linking, The water
in tank 7 may have an additive which reduces the surface
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1 tension of the water, so that it can readily creep and
2 flow into all the voids and spaces between the loops, and
3 particulaxly in lower lays of the elongated stock as wound
4 on drum 6. It should be noted, however, that dipping of
the jacketed product into water may not be necessary in
6 some cases as will be explained later.
8 As outlined ahove, the catalyst can be sprayed
9 onto the extruded jacket. As a general rule, the catalyst
should be applied subse~uent to the diffusion of the silane
11 solutiorl into the polymer powdcr particles. If, as is
12 basically feasible, the catalyst is one of the additives
13 for the silane solution, the process is simpli~ied, but
14 slnce the catalyst is the prime factor in determining speed
1~ of cross-linking, some premature cross-linking is invited.
16 Thus, it is in fact preferred to keep the catalyst separated
lq from the silane solution and to add the catalyst after the
18 diffusion of silane has been completed. Upon spraying the
19 catalyst onto the extxuded jacket, it is actually applied
even after yxafting. That, however, is not necessary. A
21 further alternative of applying the cata:lyst to the prin-
22 ciple mixture is by spraying it into the hot mass inside
~3 of the extruder.
24
25 In accordance~with the preferred form of prac- ~ ~
26 ticing the invention, one proceeds as follows regarding ~ ~ ;
27 the adding of the condensation catalyst. A batch is
28 produced in which the powdery polymer must have the same
29 consistency as the polymer used ~or diffusion with silane.
The polymer to be impregnated with catalyst must be at
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1~5~7~
l least compatible with the polymer with diffused silane.
2 If the latter is polyethylene, the former shou]d be for
3 example wax, a polyethylene with different Mol weight, an
4 ethylene mixture polymerisate or polyolefine such as
isobutylene or polyisobutylene. This polymer is now mixed
6 with the catalyst as a ratio of 100 parts polymer (e~g.
~ polyethylene) to 0.5 to lO parts (by weight) oE catalyst.
8 Preferably one produces a 1% polymer-catalyst (dry) fluid.
9 Preferably one will use dibutyltin dilaurate as catalyst.
ll Agitation and/ox heating may raise the tempera-
12 ture also here, but only to about 60 to 100C to enhance
13 difusion of the catalyst into the polymer particles.
la~ IIowev~r, room temperature may sufice. If the catalyst
lS di~fuses at elevated temperature, that subprocess is com-
16 pleted within a few minutes, and this particular powder
lq is also available now as a fluidizable batch.
~8
19 This polymer plus catalyst batch can be blended
20 with the polymer plus silane batch in the mixing chamber
21 used for the latter, as the last phase of fluidization and
22 agitati.on thereo~. The catalyst and the silane have actu-
23 ally mirlimal contact here. Alternatively, one can charge
24 the extruder with the two different batches, but preblending
25 is preferred.
26
27 It should be noted, that the catalyst-to-polymer-
Z8 to-silane ratio can be paxticularly accurately metered if
2~ two batches are prepared in the stated manner. Of course,
30 the adding of thermoplastic through the c~talyst batch has
~l to be considered upon determining the total ratios and
32 precentages of the various additives.
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~L
DESCRIPTION OF DETAILS
q After having described the process layout and
8 equipment for implementation, we proceed to the descrip-
9 tion of details, particularly regarding materials involved.
It was found advantageous to use as principle thermoplastic
ll material higher-molecule polyethylene with a melting lndex ~;
12 of about 2 (MI 2). ~mployment of this partlcular material
13 has t~le advantage that the amount of silane~peroxide needed
~ can be rather small. Moreover, upon making a cross~linking
jacket as outlined above there i5 comparatively little
16 danger that the core (3) will assume an excentric disposi- -
lq tion in the jacket and also dripping and dropping of the
18 material at grafting temperature of, say 200C is avoided.
l9 If the melting viscosity is too low, bubbles could appear
upon decompositioning of the peroxide into a gaseous
21 residue. For this particular reason one wants to use as
22 little peroxide as possible and here particularly the
23 employment of 1,3-bis (tert. butyl peroxi-isopropyl) benzene.
~4 The utilization of peroxide can be reduced further if one
uses poly-functional monomers such as divinyl benzene,
26 triallylcyanurate or-methacrylate. The yield in radicals
27 is increased while formation of decomposition products is
28 minimized. It is additional~ of advantage regarding the
29 grafting, if one uses an additive that traps methyl-radicals
without, however, reducing the total yield in production of
31
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16
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1 radicals One can use hcre higher condensated aromates
2 such as anthracene, phenanthrene, naphthyleneethylene and its
3 derivatives and at a proportion such as 0.2 t~ 3.0 parts
by weight per 100 parts polyethylene.
6 The hase polymer may be supplemented by EPDM
q (ethylene~propylene-Ter polymer elastomer ~rubber or by
8 EPM (ethylene-propylene-elastomer). These rubber compounds
9 can be added to the principal polymer, such as 5 to 2C parts
for 100 parts polymer, so as to increase the melting visco-
11 sity thereof. ~gain, thls is another way of reducing the
12 possibil:ity of formation o~ bubble3 in the extruded product.
~3
~4 A numher of organic silane compound~ are known
having an unsaturated organic substitute such as vinyl,
16 allyl, etc. and hydrolizable alcoxy or aster groups at the
17 Si atom. Presence o~ an unsaturated organic substitute is
18 the prerequisi~e of gra~ting silane upon the molecules of
19 the principle polymer. On the other hand, the speed o~
cross-linking in the presence o waker, with or without
21 catalyst, is determined by the sensitivity to hydrolysi3
22 o~ the alcoxy or ester groups, i.e. the change of the latter
23 into a silanol group. It is known that vinyl-triacetoxy
24 silane changes rather rapldly, even in the presence of mere
25 traces o~ ~ O. This high speed hydrolysis makes it possible -
26 to obtain cross-linkiny at reasonable speed without catalyst
27 and even without submersion into water. The silane com-
28 pounds are added in a proportion of 0.5 to 10 parts silane ;~
2~ per 100 parts by weight base polymer (e~g. polyethylene).
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The known methods ~or cross-linking organic polymerisates
by means of ~rafting silane compounds work with peroxides or other
substances furnishing the needed radicals so that grafting can in
fact be induced. Most of the peroxidesJ however, particularly the
ones usually employed such as dicumylperoxide (DCP~ have a high cross~
linking effectiveness, i.e. the C-C cross-linking is not su~ficiently
surplanted by grafting. That in turn renders working of the plastic ;~
difficult. The inventionJ howeverJ suggests utilizing a per~ide
with a low cross-linking effectivenessJ for obtaining the desired
grafting. For exampleJ one should use an ester peroxide such as
tert.butyl peroxyneodecanoate; tert.butyl peroxypivalate, tert.butyl
peroxy-2-ethylhexanoate or tert.butyl peroxybenzoate. All of these
ester peroxides furnish radicals upon decomposing thermally; they
have only little cross-linking effectiveness, but they do induce the
grafting, The preferred grafting inducing additive in accordance with
the invention is tert.butylperoxy-isononanoate which is added at O.OS
to 0.5% (by weight) in relation to the principle fluidized polymer.
One can use also a peroxide blend as grafting initiators, for example
a mixture of 1,3 bis (tert.butylperoxy-isopropyl)benzene and tert.butyl
peroxiisononanoate. (i.e. tert.butylperoxy-3,35-trimethylexanoate~.
As anti-oxidant one may use the monomer or an oligomer of
the 2, 2, ~ trimekhyl-dihydroquinoline. This type of anti-oxidant
can readily be grafted onto polyekhylene pursuant to the radical
reaction just like the silane. These compounds contain a rather re-
active C-C double bond `~
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1 permitting graf~ing upon the hydrocarbon chain of the
2 xespective polymer. The stabilizator molecule fixed in
3 this manner to thc macxomolecule of the high polymer can-
4 not migrate nor volatili~er but provides permcment
protection against oxidation. Such anti-o~idants are ~or
eY~ample substances traded under the desiynation ~no~ ~3,
q E~lectol H and Agerite~resin D. One can also use here
8 monomer derivatives of quinoline such as 6-ethoxi-2,2,4-
9 trimethyl dihydroquinoline (also called Santoflex ~) or
l0 the 6-dodecyl-2,2,4-trimethyl dihydroquinoline (also called '~
~l Santoflex DD~. These last mentioned substance3 have the
12 advantage that they are liquidous at room temperatuxe and
13 readily difuse into the yranulated polymer particles.
to
Adding any such substance to the silane ancl/the
lG other aaditives actually improves the chemical reaction
17 of grating and, of course, one obtains stabilization against
18 oxidation. This is particularly so as these oxidants are
l9 more homogeneously introduced into the material by operation
o the principle feature of this invention. As stated above,
21 any concurring C C cross-linking is undesired, particularly
22 whcn occurring prior to extrusion. One ~7ay o~ suppres~ing
23 this cross-linking is to de-activate these C-radicals which
24 have been produced on the polyethylene chain during grafting
but which did not receive a silane molecule. Th:is de-
~6 activating may be produced by a regular hydrogen trans~ar
27 or by adding (grafting) an anti-oxidant to that reaction
28 point. The C-C cross-linking genexally occurs at a slower
29 speed than ~oth the speed of de-activating as well as the
speed of grafting, so that this particular way of avoiding
31 undesired C-C cross-linking is indeed effective. One uses
32
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1 here from 0.05 to 2.0 parts by weiyht per 100 parts o~ the
2 principle polymer, preferably however less than 0.5 pæ ts
3 by weight.
~ .
As stated above, one will use as one of the addi-
6 tives to the silane an activator such as a polyfunctional -
q monomer, e.g. triallyl cyanurate, divinylbenzene, ethylen-
~ dimethycrylate, triallyl phosphite or others. These
9 activators are added at an amount of 0.01 to 10 parts by
weight per 100 pæ ts by the principle polymer.
11 ' ' ~,, "
12 As silane or a silane compound is ~rafted upon
13 the macromolecules of pol~ethylene, the melting index drops ;~
14 rather rapidly, i.e. the properties relatiny to the fluidity
of molten polyethylene deteriorate by virtue of the yraftiny.
~6 However, extrusion is still possible. The significant drop
~ o the meltiny index results from the impediment che macro-
18 molecules of the polyethylene have incurred by virtue of
~9 the yrafting. That in turn diminishes the ability o the
material to undergo relc~ation ollowing forminy. If for
21 example a hose is extrudecl as outlined above, the chain
22 molecules will be rather strongly oriented, and impeded
23 relaxation leads to internal strain because of the orien-
24 tation so that the mechanical~properties e.g. strengkh o
such a jacketing hose may prove inade~uate. Thus, the
2~- melting index should not drop too much on account of the
~7 grafting. By way of example, one can use lower molecular
28 polyethylene having for example MI> 2. Another remedy is
29 the utilization of extxusion tools which do not provide
for a significant orientation of the extrudate. Also, the
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.
1 exit temperature of the extruded material could be as high
2 as possiblc to enhance fluidity while the extrusion can be
3 c2rried out further under mini~al stretching of the ho~e
4 as it is ~Jithdrawn e.g. by virtue of the pcassin~ throu~h
~ tube or cable core 3.
7 Stil} o~ advantage here is a preheating of the
8 conductor and a stepwise i.e. graduated coolin~ of the
9 jacketed product. In other words, quenching shoul~ be
avoided. All these measures avold e~cess-ve dropping of
11 the melting index. ~;
12
13 ~s already stated above, i~ one use .5 a tank 7
1~ into whicll the extrucled product is imn~ersed for obtaining
cro5s--linking it may be advisable to add a su~stance to
16 the ~7a~er which reduces sur~ace tension while enhancing
17 cross-linking. For example, if the jacketed cable ox tube
18 is wound on drum 6 in multiple lays access to the inner
19 lays is difficult. Particularly if the water contains also
catalyst, reducing the surface tension will more easily wet
21 the cable surface, and water will more easily enter the
22 space hetween the loops of the coiled cable and particu-
23 larly here the inner layers. Unfortunately, this method
24 still does not permit adequate control of the amount of
2~ moisture that penetrates by diffusion into the insulation.
26
27 A certain degree of control regarding the water
28 tha~ will be efective in the interior o~ the cross-linking
29 pla~tic is obtained in the following manner. Genexally
spec~king, the speed o cross-linking in the presence o
31
32
-21-
. .
l~S7b~
1 water depends on the amount of catalyst made available and
2 or the water temperature. The maximum degree of cross-
3 linking obtained depends exclusively on the amount of grafted -
silane or silane compound and on the distribution o~ the
~ active or activatable (unsaturated) silane branches. Now it
6 should be considered that for example siloxane cro~s-linking
7 has a unique reaction mechanism because it is a silanol
8 condensation reaction wherein CEI30H an~ H20 is released.
9 -Thusj one needs only little water to ~tart the reaction
which becomes self-sustaining. The water as it is being
11 used up is continuou~lyreplenishpdasa result o the silanol
12 condensation reaction. The product such as a cable or tube
13 etc. jaclceted as described, does not have to remain in the
14 wa~er tank while cross-linlcing has been completed. Just
about 10 minutes or th~reabout su~ices, and thereafter
16 one can remove khe cable or tube from thetan]c~ rrhe cross-
17 linking once initiated proceeds now of its own accoxd.
18 Thus, in lieu of a tank 7 as shown in the Figure, it may
19 be su~ficient to pass the jacketed cable through a water
trough, just to get cross-lin]cing started which will pro-
21 ceed while the cable or tube is ~ound on drum 6 and stored
22 dry. Conversely, i not su~ficient water is released by
23 internal reaction (see also the following paragraphs)
~4 temporary storage in or pa-~sing through water for a ~ ~
25 relatively short pe~iod of time may be desirable to ada ~ -
26 additional water by diffusion.
27
28 Another way of controlling the amount of watèr
29 that i3 effective on the inside o the grafted polymer is
the following. Among the additives for the principle
Sl
32
-22-
: ~' . :' ' . ' :
~ ~ ,..... .. .
, . . . .
1~)571~8
1 polymer powder one should include a compound which releases
2 water or hydrogen. This i9 to be understood generally; the -
3 release may be a chemical or physical one but should occur
only under specific conditions, such as high temperatures.
Not only can the amount of water needed be stoichiometri-
6 cally determined, but the specific process step of immersing
q the product in water may actually be omitted entirely. The
8 water is developed within the plastic and does not have to
9 be diffused into the plastic in a separate process.
10'
11 The number of points or branches for cross-links
12 in a thermoplastic material, particularly when established
13 by grafting, can be rather accurately predetermined so that
~ the stoichiometric conditions Eor the cross-llnking i.e.
th~ amount o~ water needed is correspondingly ~uantitatively
16 predeterminable. Thus, upon using additives which release
17 water, the amount of such additives needed is lilcewise an
18 ascertainable quantity. The specific quantity for such
19 additive depends on its molecular weiyht to release the
needed amount of water molecules. Thus, one may need 0.05
21 to 5 parts by weight of the water yielding additives per
22 100 parts of the prlnclple polymer. Since in some cases
23 percentages as low as 0.05 suffice, one has an additional
24 cost advantage here.
26 ~ctually, in specific cases (namely when cross- -
~7 linking is chemically a condensation reaction of silanol)
28 one H20 molecule is produced per cross-lin~ (not of the
29 water releasing additive). This H2O will by itself cause
. . .
another cross-linking reaction etc. Therefore, the water
~1 ~. .:
32
J~ -23-
, :
~ ;i'71!~8
1 to be released ~rom an additive is needed in catalytic
2 quantities only. The rest of the water needed for conden-
3 sation cross-linlcing is produced by the silanol reaction
4 as was outlined above also with re~erence to the siloxane
cross-linking process.
~ It is of particular advantage if the additive
8 that yields water will release H2O molecules at relatively
9 hi~h temperatures only, for example at the end o~ the -
grafting process (i.e. in the hottest part of the extruder,
11 just before ejecting the extrudate). Also, one can provide
12 these additives to the graft polymer togethcr with or in
13 th~ cataly9t-polymer batch.
16
1~ . , .
19
21
22
23
~4
.~
26
27
28
2g
31
32
-~3a-
.. . . .
Particular water releasing additives of interest are for
example inorganic compounds such compositions containing hydrate
e.g. aluminum oxide-hydrate, gypsum or the like. Some organic
materials are known which will release water at particular temper-
atures and are, therefore, suitable for the purpose of this invention.
For example, one can use here cyclohexandion-semihydrate or structure-
analogous compounds. One can also use short chains of po~yvinyl
/~v~y ~'Or ~ ~
! ~ alcohols such as oligomers of vinyl alcohol or beta-b~ee.~r
acids, gamma-hydroxycarbon acids or derivatives of these acids. ~ ~
Generally speaking, one can use here any condensation re-- :
action, i.e. a reaction between two components A and B which results
in a indifferent substance C and water, wherein the requlrement of
indifference is to mean that the substance does not afect detri-
mentally the cross-linking nor does it cause deterioration oE the
desired properties o the resulting extrudate. Compounds such as
free fatty acids and metal salts thereof can be used. It should be
noted here that these components are known to serve as cross-linking
catalysts, and they can also serve as catalyst, as defined above, for
the internal production of water at particular temperatures. They
have, therefore, a dual function in this case. ;
The silane-grafted polyethylene which contains water yielding
additives is extruded in 2 as a regular thermoplastic material. The
extrusion temperature can be selected so that the dehydrating temper-
ature is just
_ 24 -
1 being reached or only briefly exceeded. As a consequence,
2 water is produced internally as a result of one of the
3 chemical re~ctions as outlined ~bovP, and cross-linking ~ ;
will now proceed under influence of the previously added
or sprayed-on catalyst. It should be noted, that any
:
6 internally developed H2O is molecularly dispersed and ev~nly
7 distributed throughout the extrudate, so that cross-linking
8 points are formed everywhere. Diffusion of the catalyst
9 from the outside results in a less homogenic distribution,
because diffusion is easier into amorphous material than
11 into crystalline structure. Consequently, cross-linking
12 is obtained more homogeneously throughout when the water
13 is internally developed which is of great advantage for the
14 electrical and mechanical properties of the cable.
lG If catalysts are missing entirely in the extruded
17 mixture, cross-linking by operation of released water is
18 slower, and takes ten to twenty times longer than with cata-
. .
19 lyst. Thus, in the absence of a catalyst a piece of plastic
will cross-l1nk completely in about thirty to eighty hours.
21 However, one has to consider that a jacketed cable is not
22 completed following extrusion. One may have to wind a layer
23 of ribbon around it, provide a shield and an additional
24 jacket ekc. All these process steps take some that so that
upon comple~tion, lncluding final testing, a period of time
26 sufficient for cross-linking has in fact elapsed and cross- -
27 l inking is completed without catalytic speed up. The cross-
28 linking density is not dependent on the presence of a ~ -
2~ catalyst; a catalyst controls merely the speed of that
process. Also, the final cross-linking density is not
31 dependent on the amount of water that was released. ~he
32 sole criterium is the amount of grafted silane.
-25-
.
' :. ' '' '` ' `, . :' '
:
1~57~t~8
1 As mentioned above, one can use hydroxycarbon
2 acids as material from which to derive the water. If one
3 uses a derivative of beta-hydroxycarbon acid, one obtains
acrylacid derivates in accordance with the equation
R ~ CH -- CH2 -- COOR ~ R--CH = CEI-COOR ~ H20
6 OH
7 These derivates are structurally related to cross-linking
8 activators known as Sartomer typas and pertalning to the
9 group of methacrylic acid ester. Utilization of this
material offers the further advantage that not only can.
11 the amounts of water needed be metered quite accurately~
12 but one obtains a cross-linking ac~ivator so that one neecls
13 less peroxide.
16
17
lg
21
22
23
2~
~ :
2~ :
27
28
2~
3~ :
32
-26-
..
., ,
., . - - . ,.,.. , . : .. . .:: .. , ",
, ~ . . .. .. ... ... . .. . . .
2 ~ ~ :
3 1~)~'7~8 .: ~
EXA*IPLES -
6 -
q After having described the metes and bound~ of
8 the method in accordance with the invention, we shall
9 describe several specific examples, denoted I, II and III.
' ~
ll Example I
12
13 The mixture, ultimately procluced consisted of
14 ~he following components (% - by weiyht)
15 polyethylene (MI-8) 100%
16 2,2,4 trimethyl-dihydroquinolin 0.5%
lq tert.-butylperoxi-isonanoate 0.5%
18 1,3 bis (tert.-butylperoxi-isopropyl
benzene) 0.02%
lg
20 vinyltrimethoxisilane 2.6%
21 triallylcyanurate 0.1%
22 1/O-dibutyltindilaurate in polyethylene
23 batch 5.0%
,
24 The mixture was obtained as follows. One p~e- ;
pared a solution with vin~l~rimethoxisiIane as solvent
26 and includea all additives except the dibutyltindilaurate.
27 This solution was added at appropriate quantity to 14.25
28 kilograms polyethylene as fluidized in a dry mixer and undex
29 1700 RPM. As the temperature rised to 95C the rotation
30 and a~itation was reduced to 650 RPMs. Actually, all the
3I
32 ~ . .
.
.' '~ ' ' ~
-27-
.
... . ..
1~5~
1 diffusion was completed at that point~ Subsequently, the
2 mixer was cooled in water so that the temperature dropped
3 to 70C whereupon 750g of the 1% dibutyltindilaLlrate in a
4 polyethylene batch was blencled into th~ mixtuxe~ The-
resulting composition was filled in P~ bags for storage.
q The 1% - batch had been prepared separately.
8 For this, 14.85 Icilo~xams granulated polyethylene was
9 mixed in the or a dry mixer with 0.15 kilograms of
dibutyltindilaurate at 1700 P~PMs and ~or about 2 to 5
11 minutes. The dibutyltindilaurate diffused rather quickly
12 into the polyethylene so that after about 5 minutes the
13 agitation speed could be reduced to 650 RPM under cooling
1~ o~ water. q~his catalyst batch was thus prepared as fluid-
lB izable powder and it was added and blended with the
16 polyethylene-silane batch a~ the stated ~uantity.
1'~ ;,~.
18 An extruder with a worm or screw of ~5 mm length
19 and an ~/D ratio of 20 was used to extrude the powder blencl.
The temperatures were adjusted for extruder zones 1,~2, 3
21 and 4 to be respectively 160C, 180C, 210C and 230C.
22 'rhe e~trusion head and the mouth had the same temperature
23 o~ 230C.
Z~
The temperatuxe of the extrudate~was about 220C
26 and dwell time in the extruder was about 2.5 minutes for
27 25 RPMs o~ the screw. That period su~ficed to complete
28 the grafting to the desired degree.
29
31
32
.' ,' , ' .
~2~-
, .
:~ . .'! , , ~
1.~57~
1 The smooth extrudate had a melting index in
2 accordance with ASTM D 1238-57T of 0.5 grams/10 min~
3 The degree of cross-linking in accordance with the solvent
4 extraction test, method IEC, was 70% after 2 hours cross-
5 linking time in 100C water.
6 ~
q Example II ;
9 polyethylene (MI-8) 10~/o
10 2,2,4 trimethyl~dihydro~uinoline 0.5%
11 tert.-butylperoxi-isonanoate 0~12%
12 1,3-bis (tert.-butylperoxi~isopropyl)benzene 0.04%
13 triallyl cyanurate 0.06%
14 vinyltrimethoxlsilane 2.6%
15 dibutyltindilaurate 0-05%
16
17 Again, all additives except dibutlytindilaurate
18 were solved in the -si]ane, and the solution was added to
19 15 kilograms polyethylene granulate and mixed at 1700 RPM
20 to obtain diffusion. After reaching a temperature of ~5
21 the agita~ion speed was reduced to 650 RPM and the mixture
22 was watex cooled whereupon the batch with 12.5 grams
23 dibutyltindilaurate was added and blended. That batch had been
24 prepared as described in Example I. After about 5 minutes
25 blending the fluidized granulate was filled in polyethylene
26 bags and processed after cooling. The extrusion was carried
27 out also as described in Example I. The smooth extrudate
28 had a melting index o 0.03g/10 min. After 2 hours of
29 cxoss-linking in water at 100C, the degree of cross-linking
3G (gel portion) ~as 72%,
31
32
-29-
-
l Example III
3 polyethylene (MI-2) 10~/o
4 2,2,4 trimethyl-dihydroquinoline 0.5%
5 tert.-butylperoxi-isonanoate 0.23%
6 triallylcyanurate 0. l~/o
q vinyltrimethoxisilane 2.5%
8 dibutyltindilaurate 0~05%
g
In this case all additives (including the
ll dibutyltindilaurate) were solved in the silane, and the
12 solution was mixed ~ith 15 kg polyethylene at 1700 RPM.
1~ After the tcrnperature had risen to 95C the agitation
l~i speed was reduced to 650 ~P~ a~d the mix~ure was cooled.
15 After 5 to 10 rninutes the still fluid granulate had a ;~
.: .
16 temperature of ~0C and was filled in bags. 60 kg mix-
lq ture were made in that fashion and then worked in a 90 mm
18 long extruder screw at a 20 L/D ratio. The temperatures
l9 in the extruder were 160 in æone l and 220 in zones 2,
3, 4 as well as in the flange. Head temperature was 275C
21 and mouth temperature was 235C. The extruded mass had
22 temperature o~ 220C.
23
24 The electrical cab~e made in that manner has a
core of multiple tLn plated, copper strands with total
26 cross~section of 70 mm . ~he extruder speed was 28 RPM
27 at a withdrawal rate of 18 meters per minute. The dwell
28 time of ~he material in the extruder was 2.5 minutes and
29 the wall thickness of the jacket was 1.4 mm.
31
32
-30-
, ... . . ........ ,, , ~ . :
: ` : . ~ ' '
'7~
1 The jacket was cross-linked for 2 hours in water
2 of 100C and the resulting degree of cross-linking was 6~/o.
3 The hot-set value for 150C and 20 N/cm2 load was 75%
4 extension. After removal of the load the residual exten-
sion was 0%.
7 The following mechanical properties were mea-
8 sured. Tensile strength 17.0 N/mm2~module of elasticity
9 50% 10.5 N/mm . The elongation at rupture was 490%. After
aging for 7 days at 150 the tensile strength was still
11 16.0 N/mm and elongation at rupture was still 41~o. ;~
12 .,
13 The invention is not limited to the embodiments
1~ described above but all chanyes and modifications thereof
~S not constitutiny departures from the spirit and scope of
16 the invention are intended to be included.
lq ' ' : ;
18
19 , ~.
21
22
23
26
27
28
29
31
32
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