CA1078545A - Method for grafting silane to thermoplastic polymers - Google Patents

Abstract

Abstract of the Disclosure It is known to graft silane compounds to polymers during an ex-trusion process, in the presence of peroxides. In the known process there tends to be a non-homogenous mixture of the silane compound, the polymer, and the peroxide in the extruder barrel. As extrusion is a relatively high temperature process from about 180°C to 220°C there results localized cross-linking of the polymer and consequently the formation of a gel in the extruded product. The presence of a gel in the product tends to cause a rough surface making the product unsuitable for some applications such as electrical in-sulation. The present invention overcomes this drawback by providing in a method for cross-linking thermoplastic or elastomeric material wherein cross-linking is preceded by grafting of silane and is carried out in the presence of moistures, the combination of steps comprising: providing the thermo-plastic or elastomeric material as dry, fluid powder; agitating the powder to obtain fluidization thereof and raising the temperature by operation of the agitating; adding a silane compound to the material prior to completion of the agitation, the silane compound being of the variety that permits grafting on the molecules of said powder; providing for a temperature below the cry-stalite melting range so that the silane or silane compound as agitated to-gether with the said material fluid is caused to diffuse into the fluid particles and providing for grafting of the silane or silane compound mole-cules to the molecules of the thermoplastic or elastomeric material.

Description

~78S4S

This application is a divisional from copending Canadian Patent application Serial Number 221,364, filed March 6, 1975.
The present invention relates to the grafting of polymers for purposes of obtaining cross-linked thermoplastics or elastomers, and more particularly the invention relates to grafting a silane compound upon the not yet cross-linked mole-cules of a polymer to obtain later cross-linking in the presence of moisture. Thus, the invention relates to the preparation of plastic or elastomeric material for subsequently obtaining cross-linking, the preparation being the grating of silane upon ~ .
the polymer. The plastic material so treated is to include ole-finepolymerizates or olefine-mixed polymerizates and others, preferably polyethylene, and to be used as jacket, envelope or cover for elongated material such as electrical cable, con-ductors or tubes, etc.
The German printed patent application DAS
1>794,028 discloses grafting alkoxy-silane components on or-ganic polymerizates with subsequent curing in the presence of moisture to obtain cross-linking. An alkoxy-silane com-pound may have the structure R Si Y3 wherein R is a vinyl group or a gammamethacrylperoxipropyl group, and Y is an alkoxy group with less than six carbon atoms.

- 2 -10'78545 Particularly, this prin-ted patent application discloses grafting of timethoxy-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 C to 220 C.
Prior to extrusion, silane and peroxide are mixed with granulated polyethylene. ln fact, the polyethylene particles are surface coated with the liquidous mixture of silane and peroxide. Homogenizatlon is produced later in the extruder, which also performs the grafting. The resul-ting product is granulated again, and the grafted, granulated 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 ex-truded as a jacket around a cable, a tube or the likeO
This known method has the disadvantage that the usual extruders produce local pemature, cross-linking due ~-to non-homogeneous peroxide distribution. The formation of such gel particles is particularly disadvantageous when the resulting product is to serve as electrical insulation.
Thus~ grafting will be carried out properly only if the temper-ature profile in the barrel of the extrusion process is adequately predetermined and controlled so that graPting proper occurs only after the materials have been homogenized sufficiently.

:

.

~:`
~078545 1 As stated, the coating o the granules intro~
2 duces some inherent inhomogeneities in the distribution

3 of peroxide, and premature cross~lin~ing during cooling

4 of the granulated grafting component is detrimental to ~ this process. Therefore, cable insulation cannot readily 6 be made in that manner particularly in those cases where 7 electrical and mechanical properties are critical. ~he 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 grafted but not yet 11 cross-linked plastic can be stored only to a limited 12 extent and for a limited duration as any moisture will 13 start the cross-linking. It is apparen~, however, that 14 cross-linking should begin only after the plastic has assumed and has been yiven its final shape.

1~ Another disadvantage of the known procedure is 18 to be seen in that after the catalyst has been added (the 19 grafting component was added earlier) extrusion becornes more complicated as long as even traces of moisture are 21 present~in the material. As a consequence, the surface 22 of the product is quite rough which is a significant 23 deficiency if the product is subjected to an electric 2~ field and voltage.

~78S45 ,, 3 :
4 . :
I)ESCRIPTION OF THE INVEN?ION .
q It is an object of the present invention to 8 provide a new and improved method of grafting a silane 9 compound upon thermoplastic or elastomeric material to permit such material to cross-link in the presence of 11 moisture wh~ile avoiding the problems and interferences 12 outlined above.

14 - In accordance with the preferred embodiment o~
15 the present invention, it is suggested to provide the ~-16 thermoplastic or elastomer as dry, fluid powder which is 17 agitated for~internal, xapid, high speed, turbulent move- ;
18 ment. Silane or silane compounds and other additives, all ;
~; 19 in liquid fo~m,~are added to that powder be~ore and/or during the fluidization while subjecting the powder to a 21 higher-than-normal temperature, but below the crystallite 22 melting range (e.g. in the range from 60 to 1~0C) so - 23 that the additives diffuse at least partially into the ~ 24 surface of the fluid-particles to be distributed therein r ~_n25 as~homogeneously as possible. The thus treated material A26 does not ~oose its character as a ~luidizable powder. That 27 powder is subsequently subjected to steps by means o which 28 grafting is obtained~ It should be mentioned, that the 2~ term powder is used here in the general sense and is to include also grit like makerial and material o~ granulated ~1 consistency.

: .

, _5_ - , . . , . :.

1(~78S45 1 It can thus be seen, that prior ~o the ~inal 2 shaping extrUsion, separate extrusion for grafting and 3 subsequent granulation is not necessary; the material ~ose 4 never ~e~e~ its powdery~luidizable consistency. This is clearly simpler and more practicable.

The additives themselves are homogenized prior B to the mixing with the fluiaized or fluidizable powdcr.
9 The$e additives are the silane and/ox silane compounds as used for grafting;ree radical initiators such as per-11 oxides; poly-functional monomers as activators such a5 12 e.g. txiallyI cyanurate, divinyl benzene, ethylene 13 dime~hycrylate, triallylphosphite and others: condensation 14 catalys~s such as dibutyltindilaurate or heavy metal salts fatty of long chain / acids may be included at that point or , ~16 added later. ~hese materials are mixed, at least in parts~
17 prior to being added to the powdery thermoplastic or elas-18 tomeric material. The latter powder should be predried to lg prevent premature cross-llnking, so as to improve the quality of the productO r~he thermoplastic or elastomeric ~ol~-th~/en~
21 material could be (preferably) po~thïlcno or a copolymer-22 izate of ethylene with another co-monomer such as ethylene-23 vinylacetate; propylene; butylene, etc.

The grafting proper can be obtained in various 26 ways. By way of example, the silane or silane compound 27 can be gr~tedon the macromolecules of khe polyethylene 28 under mechanical-thermal treat-merlt e,y. while being form-2~ shaped in an extruder. ~he treal;menk requires kemperatuxes between 160 and 250C; khat ranye depends e~entiall~ on ;
,.. .
~ ~6-, . .. . .
. ~ '. ' ' .

. ~07854~ii 1 the decomposition kineticsof the particular additives that 2 is to ~urnish the active radicals. Also, d~lell time in 3 and throughput of the extruder are actors which determine 4 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 9 and the quality of the resulking product is even higher.
.
The melting temperature may be less than usual, e.g. below 11 180C so that the entire thermal operating range to which 12 the material is sub]ected is lower. By way of example, 13 one may use here a Van-de-Graaf accelerator emitting elec-14 trons at a doses of 0.1 to 3 l~rd, ater having extruded the material into the desired shape. This was particularly 16 used ~or Example II (infra).
~7 18 It may be advisable to include among the additives an 19 anti-oxidant for thermal stabilization. ThiS is particularly advantageous if the anti-ox~idant is of the variety which 21 influences very little the reaction involving the radicals.
22 Rather, they are included in the grafting and are being 23 grafted on the polymer macromolecu]es. -In the preferred form of practiciny the invention 26 the condensation catalyst will be added after the silane 27 plus additives have diffused into the particles of the 28 ~luidizable polymer. By way of example, a batch is pro~
29 pared of the same granulated powdery polymer and mixed with the condensation catalyst. ~hat ba~ch is ~hen added to the ~.
" ' - :,:

:
'~'. " : ' 8~5 1 fluidized polymer in the last phase o agita~ion or in the 2 initial, charge phase of subsequcn~ cxtrusion. Alterna~
3 tively, the catal~st can be sprayed into the extruded substance or onto the extruded product.

;, .

"
:
:

.

` ` 1~78S4S

The present invention provldes a methad for cross-linking thermoplastic or elastomeric material, wheTein cross-linking is preceded b~ gxating a silane containing an unsaturated organic substituent and is carried out in the presence of moisture, the combination of steps com-pYiSing: providing the thermoplastic or elastomeric material as dry, fluid powder; agitating the powder to obtain fluidization thereof and ~ai~ing the temperature by operation of the agitation; adding as a liquid said silane compound to the thermoplastic or elastom~ric material prior to completion of the agitation; providing for a temperature above about 60C but below the crystallitemelting range of said thermoplastic or elastomeric material so that the graft compound as agitated together with the said thermoplastic or elastomeric fluid par~icles is caused to homogeneously diffuse into the fluid particles; and subsequently providing for grafting of the graft compound molecules to the molecules of the thermoplastic or elastomeTic material.
~ '`' .
': ..
':

~ -8a-1071~5~5 : 3 DESCRIPTION OF THE DR~WI~GS
6 . .
q 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 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-A 13 cription taken in connection with the accompanying 14 in which.
; 15 16 The Figure shows schematically the carrying out 17 of the process in accordance with the pr~ferred embodiment.

19 Reference numeral 1 denotes a high speed mixer being, for example, a vessel in which cutters, blades, 21 vanes ox the like rotate at high speed, such as 500 to 22 3000 RPM. Polyethylene if used as starting material is 23 charged to that mixer. Reference numeral 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 act sufice for 2~ drying.

' 9_ :~ ~ ,. . ............ . ... . ..
~ .. . . . . . .. . . ...
.

`` 1~78545 .
- 1 The mixing speed is selectéd in accordance with 2 the mesh size of the particles. While nothing precludes 3 the application of external heat, it was ~ound to be suf-4 ficient to run the high speed agitakor at such an intensity that particle coll~sion and friction actually suffices to 6 heat the fluidized powder and granulated polymers. Since the 7 sur~ace of the individual particles may begin to soften, 8 agitation is still continued, though possibly at a 9 reduced ratelbut sufficent to pr~vent agglomeration caking and clustering of particles.
11, ' 12 The purpose of softening and melting the particle 13 surface is to enhance the later ensuing diffusion and to the 14 increase/diffusion rate. Since the diffusion is the prin-ciple purpose of agitation, raising the temperature is quite 16 important~ High speed agitation may not suffice so that 17 additional heating may be needed. The agitation of course 18 will be highly instrumental in rapidly equalizing the tem~
19 perature throughout the fluid material so that thermal conduction occurs only from particle to particle and inside 21 of each particle, but the mixe~-induced turbulence equalizes 22 the temperature in and throughout the vessel.

24 As high speed agitation and fluidization takes place the needed additives e,g. silane and peroxide and 26 other additives are fed to the mixer. These additives are 27 in the liquid state, but due to the evacuation a considerable 2B portion will evaporate immediately ~o that only little li~uid 29 remains~ The temperature of the material may have been xaised to 80 to 100C, the su~ace o~ the particles has . '~

.
.

78S~5 1 soitened so that the additives diffuse readily into the 2 powder or granulate particles. The di~fuslon 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 separate and individual ele-6 ments of the ~luid and do not agglomerize while the q 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. Moreover, the additives 12 become rather-homogenically distributed, not only throughout ~ -13 the fluidized material as such, but also within each particle.
14 The process,thus,is continued for a period which is known in advance, namely the time it takes for the additives to diffuse 16 deeply into the particles and is more or less homogenically 17 distributed therein. ~lowever, the agitation speed may have 18 been reduced as it takes more mixing power and agitation to 19 raise the temperature than to )~eep the temperature constant or approsimately constant for difusion~ The continu~ed 21 agitation is needed to prevent the rather warm particles 22 from clustering. Also, the gaseous phase should participate 23 in the diffusion so that the maintaining of the fluidized 24 bed is desirable throughout. Unlike the known methods, the particles are not just coated but impregnated thoroughly by 26 the additives. ~ ¦
27 ~
28 As outlined earliex, the additive~ ~hich are 29 caused to diffuse by th~ agitation process, have been mixed with and/or solved in the liquidous æilane or silane .... . .

7~5~ ~

1 compound which acts, there~ore, as solvent for peroxide, 2 activators, and anti-oxidant. I~e catalyst could be added 3 to that solu tion, but its addiny to the mixture is pre-4 erably deerred.

q , :

1~ ' .

, .: .
` 28 .
_~la-.

. - . ~ .

:

These additives will therefore also diffuse into -the par-ticles of the fluidized polye-thylene powder and -the resulting homogeneity in the distribution will depend on the homogeneity of the mixture, bearing in mind that the agita-tion will also homogenize these additives even before diffusion begins.

The solution of additives could be added to the ;-~
powder even prior to high agitation, even when the fluidized polyethylene is still cold. However, in order to obtain a rapid onset of diffusion, a high diffusion temperature and a large (initial) diffusion gradient should concur, so that the adding of the additives may be deferred until heating and/or agitation has raised the temperature to about 80C. However, it 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 diffu-sion in the powdery polymer. The condensation catalyst ;

could be added at that point, but that aspect shall be described later.

' ' "' . ' ' :' ' ' ,'~ , :., " 1~78S45 Upon completion of the agitation, an extruder 2 is charged with the content of the mixing vessel. The extruder 2 is of conventional variety and of the type used to extrude granular substances to jacket a cable or a tube.
The cable or tube 3 to be jacketed thus runs through the extruder and a jacket is extruded around it. The extruder is operated at a temperature of the extruded material to be in the range from 160 to 250C, preferably from 190 to 230C. That extruder does not have to perform homo-genization work. Therefore, one does not need specialworms or screws which perform in parts mixing of the raw charge. The extruder (barrel) chamber is however, the location in which grafting occurs, just prior to jacketing the cable or tube. ~i-The jacketed cable or tube 4 as it leaves the extruder passes, for example, through a spray device 5 by means of which the catalyst (for cross-linking) is applied.
Thus, in this form of practicing the invention the catalyst 20 is not par-t of the additives. The catalyst is therefore in ~-contact with the silane for a minimum period of time, just ahead of the cross-linking. This way, the speed of cross-linking once initiated by application of moisture is rather high.

The cableissubsequently wound on a drum 6 which may be finally placed into a tank of water, 7, and stays therein for a perlod sufficient for the water to diffuse into the envelope so as to obtain cross-linking. The water in tank 7 may have an additive which reduces the surface - 13 ~

. .
', "'"' ' ' ~ ' ~

~(~713545 1 tension of the water, so that it can readily creep and 2 flow into ail the voids and spaces between the loops, and 3 particularly 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 above, the cataIyst can be sprayed 9 onto the extruded jacket. As a general rule, the catalyst should be applied subsequent to the diffusion of the silane , 11 solution into the polymer powder particles. If, as is 12 basically feasible, the catalyst is one of the additives 13 for the silane solution, the process is simpliied, buk 14 since the catalyst is the prime factor in determining speed 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 extruded jacket, it is actually applied ~20 even after grafting. That, however, is not necessary. A
21 further alternative of applying the catalyst to the prin- -22 ciple mixture is by spraying it into the hot mass inside 23 of the extruder.

: ~ .
~ 25 In accordance with the prefexred form of prac-a 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 difusion w~th silane.
The polymer ~o be impregnated with catalyst must be at , ' ' . .

- : , , ~ , ; .

10~8S45 l least compatible with the polymer with difused silane, 2 If the latter is polyethylene, the former should be ~or 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 lO0 parts polymer (e.g.
polyethylene) to 0.5 to lO parts (by weight) of catalyst.
8 Preferably one produces a 1% polymer-catalyst (dry) fluid.
9 Preferably one will use dibutyltin dilaurate as catalyst.
10' 11 Agitation and/or heating may raise the tempera-12 ture also here, but only to about 60 to 100C to enhance~
13 diffusion of the catalyst into the polymer particles.
14 However, room temperature may suffice. If the catalyst 15 diffuses at~elevated temperature, that subprocess is com-16 pleted within a few minutes, and this particular powder 17 is also available now as a fluidizable batch.
.

19 This polymer plus catal~st 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 agitation thereof. The catalyst and the silane have actu-23 ally minimal contact here. Alternatively, one can charye 24 the extxuder with the two different batches, but preblending 25 is preferred.

27 It should be noted, that the catalyst-to~polymer-28 to-silane ratio can be particularly accuràtely metered i~
29 two batches are prepared in the stated manner. Of course, 30 the adding o thermoplastic ~hrough the catalyst batch has 31 to be considered upon determining the total xatios and 32 precentages o~ the various additives.
, .
, .

~ 785~S
.
.
i ~ .

4 :
DESCRIPTIO~ OF DE~A ILS
: 6 After having described the process layout and 8 equipment for implementation, we proceed to the descrip-9 tion of details, particularly regarding materials involved.
- 10 It was found advantageous to use as principle thermoplastic 11 material higher-molecule polyethylene with a melting index ~ ~ 12 of about 2 (MI 2). Employment of this particular material i ~I3 has the advantage tha-t the amount of silane~peroxide needed 14 can be rathç~ small. Moreover, upon making a cross-linking ~` ~ 15 jacket as outlined above there is comparatively little , ~ 16 danger that the core (3) will assume an ~ disposi-17 ~tion in the jacket and also dripping and dropping of the 18 material at grafting temperature of, say 200C is avoided.
19 If the melting viscosity is too low, bubbles could appear 20~ upon decompositioning o 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.

24 The utilization of peroxide can be reduced further if one ~25 ~uses poly-functional monomers such as divinyl benzene, 26 triallylcyanurat2 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 ~ield in production o .. . -~

1(~785~5 radicals. One can use here higher condensated aromates such as anthracene, phenanthrene, nzphthyleneet~ylene and its derivatives and at a proportion such as 0~2 to 3.0 parts by weight per 100 parts polyethylene.
The base polymer may be supplemented by EPDM (ethylene-propylene-Ter polymer elastomer ~rubber)) or by EPM (ethylene-propylene-elas~omer).
These rubbeT compounds can be added to the pTincipal polymer, such as 5 to 2Q pa~ts for 100 pa~s polymer, so as to increase the melting viscosity thereo. Again, this i5 another way of reducing the possibility of formation o bubbles in the extruded product.
~ number o organic silane compounds are known having an un-~aturated organic substitute such as vinyl, allyl, etc. and hydrolizable alkoxy or ester groups at the Si atom. Presence of an unsaturated organic substitute is the prerequisite of grafting silane upon the molecules of the p~inciple polymer. On the other hand, the speed of cross-linking in the presence of water, with or without catalyst, is determined by the sen5itivity to hydrolysis of the alkoxy or ester groups, i.e. the change o the latter into a silanol group. It is known that vinyl-triacetoxy silane changes rather xapidly, even in the presence of mere traces of H20.
This high speed hydrolysis makes it possible to obtain cross-linking at reasonable speed without catalyst and even without submersion into water.
The silane compounds are preferably added in a proportion of 0.5 to 10 parts silane per 100 parts by weight base polymer (e.g. polyethylene).

~17 107~35~5 1 The kno~n methods ~or cross-linkiny organic 2 polymerisates by means of grafting silane compounds work 3 with peroxides or other substances furnishing the nee~ed radicals so that graftiny can in act be induced. ~ost of the peroxide~, however, particularly the ones usuall~
6 employed such as dicumylperoxide (DCP) have a high cross-7 linking effectiveness, i.e. the C-C cross-linl~ing is not 8 sufficently surplanted by grafting. That in turn renders 9 workiny of the plastic difficult~ The invention, however, suggests utiliziny a peroxide with a low cross-linlciny 11 effectiveness, for obtaining the desired grafting. For 12 ex~nple, one should use an ester peroxide such as 13 tert.but~l perox~neoaecanoate; terk.butyl peroxypivalate, 14 tert.butyl peroxy-2-ethylhe~anoate or tert.butyl perox~~
benzoate. All of these ester pero~ides furnish radicals 16 upon decomposing thermally; they have only little-cro~s-17 linking effectiveness, but they do induce the grafting.
18 The preferred grafting inducing additive in accordance 19 with the invention is tert.butylperoxy-isonanoata which is added at 0.05 to 0.5% (by weight) in relation to the 21 principle fluidized polymer. One can use also a peroxide ~2 blend as grafting initiators, for exampLe a mixture of '` iS~rc,p~ 1 1,3 bis (tert~butylperoxy-i30plpy~)benzene and tert.butyl 24~ peroxiisonanoate.(i.e. tert.butylperoxy-3,35 ~ .

26 As anti-oxiaant one may use the monomer or an 27 oligomer of the 2, 2, 4 trimethyl~dihydroquinoline. Thi~ ~

28 type o anti~oxidant can readily begxafted onto polyethylene 29 pursuant to the radical reaction ju5t likQ the silane.

These compounds contain ~ rather reactive C~C double bond .

-18~
': ~ , :' :

~C~7~545 1 permitting grafting upon the hydrocarbon chain of the 2 xespective polymer. The stabilizator molecule fixed in 3 this manner to the macromolecule of the high polymer can-4 not migrate nor volatilize, but provides permanent prQtection against oxidation. Such anti-o~idants are ~or 6 es~ample subst2nces traded under the desiynation ~nox HB, 7 Flectol H~and Agerite resin ~ One can also use here 8 monomer derivatives of quinoline such as 6-ethoxi~2,2,4-9 trimethyl dihydroquinoline (also called Santofl~x A~ or the 6-dodecyl-2~2~4-trimethyl dihydroquinoline (also called 11 Santoflex D ~ . These last mentioned substances have the 12 advantage that they are liquidous at room temperature and 13 readily diffuse into the granulated polymer particles.

to Adding any such substance to the silane and/the 16 other additives actually improves the chemical reaction 17 of grafting and, of course, one obtains stabilization against 18 oxidation. This is particularly so as these oxidants are 19 more homoyeneously introduced into the material by operation of the principle feature of this invention. As stated above, 21 any concurring C-C cross-linking is undesired, particularly 22 when occurring prior to extrusion. One way of suppressing 23 this cross-linking lS to de-activate khese C-radicals which 24 have been produced on the polyethylene chain during graftiny but which did not receive a silane molecule. This de-26 activating may be produced by a regular hydrogen transfer 27 or by adding (grating) An anti-oxidant to that reaction 28 point. The C-C cross-linking gen~rally occurs at a ~;lower 2~ speed than both the speed of de-activating as well as the speed of grafting, so that ~his particular way o~ avoidin~
31 undesired C-C cross~ ing is indeed effective. One U5eS

~ . - . .
--1~-- .
.

- ~j 107~35~5 1 here from O~OS to 2.0 parts by weight per 100 parts o~ the 2 principle pol~mer, preferably however less than O.S parts 3 by weight.
As stated above, o~ne will use as one of the addi-6 tives to the silane an activatox such as a polyfunctional ,~
monomer, e.g. triallyl cyanurate, divinylbenzene, ethylen-~8 dime~hycrylate, txiallyl phosphite or others. These 9 activators are added at an amount o 0.01 to 10 parts by weight per 100 parts by the principle pol~mer.

12 A~ silane or a silane compound is grated upon ~3 the macromolecules of polyethylene, the melting index drops 1~ rather rapidly, i.e. the properties relating to the fluidiky o~ molten polyethylene deteriorate by virtue of the gra~ting.
16 However, extrusion is still possible. The signi~icant drop 17 of the melting index results from the impediment the macro- -18 molecules o the polyethylene have incurxed by virtue of 19 the grating. That in turn diminishes the ability of the .
~ ~ 20 materlal to undergo rela~ation following forming. If for .,.
~ example a hose is extruded as outlined above, the chain 22 molecules will be rather strongly oriented, and impeded 3 relaxation~leads to internal strain because o~ the orien-24 ta~ion so that the mechanical properties e.g. strength of 25 ~such a ~aclseting hose may prove inadéquate. Thus, the 26 melting index should not drop too much on account of the 27 graf~ing. By way of example, one can use lower molecular 28 polyethylene having ~ox example MI> 2. Another remedy is 29 the utilization of extrusion tools which do not provide for a siyniicant oxientation of the extrudate, ~lso, the , . , .; : . ' , ' ': ' ~

1 exit temperature of the extruded ma~erial could be as high 2 as possible to enhance fluidity while the extrusion can be 3 carried out further under minimal stretching of the hose as it is withdrawn e.g. by vixtue of ~ e passing through tube or cable core 3.

Still of advantage here is a preheating of the 8 con~uctor and a stepwise i.e. graduated cooling of the 9 jaclceted product. In other ~10rds, quenching should be avoided. All these measures avoid excessive dropping of ll the melting index. -~

13 As already s~ated ahove, if one uses a tank 7 l4 into which the extruded product is im~ersed for obtaining cross-linking it may be advisable to add a substance to 16 the ~ater which reduces surface tension while enhancing 17 cross-lin~;ing. For example, if the jacketed cable or tube 18 is wound on drum 6 in multiple lays access to the inner ~l9 lays is dif~icult. Particularly if the water contains also catalyst, reducin~ the surface tension will more easily wet 21 the cable surface, and water will more easily enter the 22 space between 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 26 ~oisture that penetrates by diffuslon into the insulation.
; 26 27 ~ certain degree of control regaxding the water 28 that will be effective in the interior of th~ cross-linking 29 pl~stic is obtained in the ollowing manner. Generall~
speaking, the speed of cros~linking in the pxesence a~

.

- : -, : .
.

1C~78S45 1 water depends on the amount o~ catalyst madc available and 2 ox the water temperature. The maxIm~n degxee o~ cross 3 linking obtained depends exclusively on the amount of yrafted 4 silane or silane compound and on the distribution of the ~ ac~ive or activatable (unsaturated) silane branches ~0~`7 it 6 should be considered that for example siloxane cross-lin!{iny q has a unique reaction mechanism because it is a silanol 8 condensa~ion reaction wherein cH30H and H20 is released.
9 Thus, one needs only little water to staxt the reaction which becomes-self-sustaining. The water as it is being 11 used up is continuou~lyreplenish~dasa result of the silanol 12 condensation reaction. The product such as a cable or tube 13 etc. jacketed as described, does not have to remain in the 14 water tank ~hile cross-linlsing has been completed. Just about 10 minutes or thereabout suffices, and thereafter 16 one can remove the cable or tube from thetank. The cross-1~ linking once initiated proceeds now of its own accord.
18 Thus, in lieu of a tank 7 as shown in the Figure, it may 19 be sufficient to pass the jacketed cable through a water txough, just to get cross-linl~ing started which Will pro-21 ceed while the cable or tube is wound on drum 6 and stored 22 dry. Conversely, i not sufficient water is released by 23 internal reaction (see also the following paragraphs) 24 temporary storage in or passing J~hrough water for a relatively short pe~iod o time may be desirable to add 26 additional water by difusion.

. .

1~7~S~

~ EYAMPLES
q After having described the metes and bounds o~
8 the method in accoxdance with the invention, we shall 9 describe several specific exa~ples, denoted I, II and III.

11 Example I

13 The mixture, ultimately produced consisted of I~ the following components (% - by weight) 15 polyethylene (MI-8) 100%
16 2~2~4 trimethyl-dihydroquinolin 0.5%
17 ter~.-but~lperoxi-isonanoate 0.5%
18 1,3 bis (tert.-butylpexoxi-isopropyl benzene) 0.02%

20 vinyltrimethoxisilane 2~6%
21 triallylcyanurate 0.1%
22 1%-dibutyltindilaurate in polyethylene batch 5.0%

24 The mixture was obtained as follows. One pre-25 ~pared a solution with vinyltrimethoxisilane as solvenk 26 and includ~d all additives except the dibutyltindilaurate.
27 This solu~ion was added at appropriate quantiky to 14.25 28 kilograms polyethyl~ne as ~luidi~ed in a dxy mixer and under 29 1700 RPM. As the temperatuxe xised to 95C the rotation and agitation was reduced to 650 RPMs. Ackually, all the _23_ ;

: . ' : .: .

1~785~5 1 diffusion was cornpleted at that point. Subsequently, the 2 mixer was cooled in water so that the temperature dropped 3 to 70C whereupon 750g of the 1% dibutyltindilaurate in a polyethylene batch was blended into the mixture. The resulting composition was filled in PE bags for storage.

q The 1% - batch had been prepared separately.
8 For this, 14.85 Icilograms granulated polyethylene was 9 mixed in the or a dry mixer with 0.15 kilograms of dibutyltindilaurate at 1700 RPMs and for abo~t 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~ of water. ThiS cata]yst batch was thus prepared as fluid-izable powder and it was added and blended with the 16 polyethylene-silane batch at the stated quantit~.
lrt 18 An extxuder with a worm or screw o~ 45 mm lerlgth 19 and an L/D xatio of 20 was used to extrude the powder blend.
The temperatures were adjusted for extruder zones 1, 2, 3 21 and 4 to be respectively 160C, 180C, 210C and 230C.
22 The extrusion head and the mouth had the same temperature 23 of 230C.

The temperature of the extrudate was about 220C

26 and dwell time in the extnlder was about 2.5 minutes for 27 25 RPMs of the screw. That pexiod sufficed to complete 28 the gra~ting to the desixed degree~

~2~-.~ ' , ' , ' .

1~)785~5 1 The smooth extrudate had a melting index in 2 accordance with ASTM D 123~-57T of 0.5 grams/10 min.
3 The degree of cross-linking in accordance with the solvent extraction test, method IEC, was 70% after 2 hours cross-~ linking time in 100C water.

q Example II

9 polyethylene (MI-8) 10~/o 10 2,2,4 trimethyl~dihydroquinoline 0.5%
11 tert.-butylperoxi-isonanoate 0.12%
12 1,3-bis (tert.-butylperoxi-isopropyl~benzene 0.04%
13 triallyl-cyanurate 0.06%
1~ vinyltrimethoxisilane 2.6%
15 dibutyltindilaurate 0.05%

17 Again, all additives except dibutlytindilaurate 18 were solved in the -silane, and the solution was added to 19 15 kilograms polyethyl~ne granulate and mixed at 1700 RPM
20 to obtain diffusion~ After reaching a temperature of 95 21 the agitation speed was reduced to 650 RPM and the mi~ture 22 was water cooled whereupon the batch with 12.5 grams 23 dibutyltindilaurate was added and blended. That batch had been 2~ prepared as described in Example I. After about S minutes 25 b~ending 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. A~ter 2 hours o~
29 cross-linking in water at 100C, the degree o~ cxoss-]inking 30 (gel portion) was 72%~

.
_25_ .: . : ., ' ..

~.~7~3S~5 1 ExamPle III

3 polyethylene ~MT-2) 10~/~
4 2,2,4 trimethyl-dihydroquinoline 0.5%

5 tert.-butylperoxi-isonanoate 0.23%

6 triallylcyanurate O. 18%

7 vinyltrimethoxisilane 2.5%

8 dibutyltindilaurate 0.05%

In this case all additives (including the 11 dibutyltindilaurate) were solved in the sllane, and the 12 solution was mLxed with 15 kg polyethylene at 1700 RPM.
13 After the temperature had risen to 95C the agitation 14 speed was reduced to 650 RPM and the mixture was cooled.
15 After 5 to 10 minutes the still fluid granulate had a 16 temperature of 70C and was filled in bags. 60 kg mix-17 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 19 in the ex~ruder were 160 in zone 1 and 220 in zones 2, 3, 4 as well as in the ~lange, Head temperature was 275C
21 and mouth temperature was 235C. The extruded mass had 22 temperature of 220C.

24 ~he electrical cable made in that manner has a core of multiple tin plated, copper strands with total 26 cross-section of 70 mm . The extruder speed was 28 RPM
27 at a withdrawal rate of 18 meters per minute. The dwell 28 time of the material in the extruder wa~ 2.5 minutes and 29 the wall thickness o~ the jacket was 1~4 mm.

_26_ ~78~5 1 Th~ jacket was cross-linked for 2 hours in wat~r 2 of 100C and the resulting degree of cross-linking was 6~/o.
3 The hot-set value for 150C and 20 N/cm load was 75%
4 extension. After removal of the load the residual exten-sion was 0%.

q The following mechanical properties were mea-8 sured. Tensile strength 17.0 N/mm ;module of elasticity

9 50% 10.5 N/mm . The elongation at rupture was 490%. A~ter aging for 7 days at 150 the tensile strength was still 11 16.0 N/mm2 and elongation at rupture was still 410/~.

13 The inv~ntion is not limited to the embodiments 14 described above but all changes and modifications thereof not constituting departures from the spirit and scope of 16 the invention are intended to be included.

2~
~5 32 ;

_27_ .

Claims (35)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for cross-linking thermoplastic or elastomeric material, wherein cross-linking is preceded by grafting a silane containing an un-saturated organic substituent and is carried out in the presence of moisture, the combination of steps comprising: providing the thermoplastic or elastomeric material as dry, fluid powder; agitating the powder to obtain fluidization thereof and raising the temperature by operation of the agitation; adding as a liquid said silane compound to the thermoplastic or elastomeric material prior to completion of the agitation; providing for a temperature above about 60°C but below the crystallite melting range of said thermoplastic or elastomeric material so that the graft compound as agitated together with the said thermoplastic or elastomeric fluid particles is caused to homogeneously diffuse into the fluid particles; and sub-sequently providing for grafting of the graft compound molecules to the molecules of the thermoplastic or elastomeric material.

2. In a method as in claim 1, wherein the graft compound is added at a temperature below the crystallite melting range of the said powder.

3. In a method as in claim 1, wherein the graft compound is added at a temperature between 60° and 100° C.

4. In a method as in claim 1, wherein the grafting concurs with mechanical-thermal treatment of the powder.

5. In a method as in claim 4, wherein said treatment is carried out at a temperature between 160°
and 250°C.

6. In a method as in claim 4, wherein the powder is extruded.

7. In a method as in claim 1 wherein the graft-ing is obtained by subjecting the mixture to high energy radiation.

8. In a method as in claim 7 wherein the material has a temperature below 180°C during said radiation.

9. In a method as in claim 1 wherein the silane compound is added to the powder prior to agitation.

10. In a method as in claim 1 wherein the silane compound is added when the temperature of the powder has risen to 80° to 100°.

11. In a method as in claim 1 wherein the silane compound is added when the temperature of the powder has risen to 80° to 90°.

12. In a method as in claim 1 wherein the tem-perature as provided is between 80° and 100°C during which the said diffusion occurs.

13. In a method as in claim 1, and including pre-drying the powder before adding the silane compound.

14. In a method as in claim 1, wherein the silane compound is added to the powder under conditions of low pressure to obtain evaporation.

15. In a method as in claim 14, wherein the diffusion occurs from the liquidous as well as from the gaseous phase of the silane compound.

16. In a method as in claim 1, wherein the mix-ture is formed and subsequently placed in intimate contact with water.

17. In a method as in claim 16, and including adding to the water a substance which reduces surface tension.

18. In a method as in claim 1, wherein the silane compound is mixed with peroxide.

19. In a method as in claim 18, wherein the silane compound is additionally mixed with an activator.

20. In a method as in claim 18, wherein the graft compound is a silane compound mixed with an anti-oxidant.

21. In a method as in claim 1, wherein the graft compound is a silane compound and a condensation catalyst is added to the mixture subsequent to diffusion.

22. In a method as in claim 21, wherein the condensation catalyst is added under temporary continuation of the agitation.

23. In a method as in claim 21, wherein the mixture is extruded, the catalyst being added to the hot mixture as it is being extruded.

24. In a method as in claim 21, wherein the mixture is applied to an elongated object, the catalyst being sprayed on.

25. In a method as in claim 21, wherein a batch of the powder and of the catalyst is prepared to obtain diffusion of the catalyst into the latter powder and adding the batch as mixed to the mixture.

26. In a method as in claim 1, wherein the silane is added in an amount of from 0.5 parts to 10 parts by weight of said silane per one hundred parts by weight of the base polymer.

27. In a method as in claim 1, wherein the silane compound is mixed with a anti-oxidant.

28. In a method as in claim 27, wherein the oxidant is a monomer or oligomer of the 2,2,4 trimethyl-dihydroquinoline.

29. In a method as in claim 27, wherein the anti-oxidant is also grafted onto the polymer.

30. In a method as in claim 1, wherein 0.05 to 0.5 parts per 100 parts (by weight) of powder, of a peroxide is added to the silane as solution therein.

31. In a method as in claim 30, wherein the peroxide is an ester-peroxide.

32. In a method as in claim 31, wherein the esteroxide is tert.-butylperoxy-isonanoate.

33. In a method as in claim 30, wherein the peroxide is a blend of 1, 3 bis(tert.-butylperoxy-isopropy) benzene and tert.-butylperoxy-3,3,5-trimethylhexanoate.

34. Method as in claim 1, wherein the silane compound is vinyltriacetoxy silane.

35. Method as in claim 1, wherein the fluid is higher molecular polyethylene with a melting index of about 2.