CA1066010A

CA1066010A - Manufacture of extruded products

Info

Publication number: CA1066010A
Application number: CA241,176A
Authority: CA
Inventors: Peter Swarbrick; William J. Green; Charles Maillefer
Original assignee: Maillefer Sa Ets; BICC PLC
Current assignee: Maillefer Sa Ets; Balfour Beatty PLC
Priority date: 1974-12-06
Filing date: 1975-12-05
Publication date: 1979-11-13
Also published as: US4117195A; AU503860B2; NL186096B; AU8727975A; FR2293776B1; SE7513728L; NL186096C; CH627402A5; FR2293776A1; BE836338A; NO144423B; NO754113L; AT345554B; JPS5825583B2; CH634253A5; NO144423C; US4117195B1; DE2554525A1; ES443269A1; DE2554525C2

Abstract

A B S T R A C T In a method of making a cross-linked extruded product from polyethylene of other suitable polymer, the polymer is metered into a screw extrusion machine together with compound-ing ingredients comprising a hydrolysable unsaturated silane, a free-radical generator and a silanol condensation catalyst. The compounding ingredients are blended with the polymer in the barrel of the extruder and the mixture is heated sufficiently to effect grafting of silane groups to the polymer, the amount of free-radical generator being sufficiently low to limit direct free-radical cross-linking to a level that will not prevent ext-rusion of the material. The reaction mixture is extruded directly from the same extruder through an extrusion die to form an elong-ate shaped product and cross-linking is effected by subjecting the shaped product to the action of moisture.

Description

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Thi9 invention relate~ to the manufacture of extruded product~, eqpecially but not exclus~vely electric cable~ and pipes, that are of carbon-chain polymer~ cross-linked by the use Or hydrolysable unsaturated silane. Such product~ are formed by ~ir~t reacting the polymer with the hydroly~qable unsaturated silane in the presence of a free-radical generator, such a~ a peroxide, and afterwards expo~ing the grafted material to the e~fects of moisture and a silanol condensation catalyst. Suitable reagentq have been de~cribed in the specification of British Patent No. 1286460. It i9 n~w knownJ
however, that the process is applicable to a variety of polymers other than polyethylene and the modified polyeth~lenes referred to in that specification, for example chlorinated polyethylenes and a wide range o~ olefin copolymers can be processed, and in the practice of the present invention the reaction conditions set forth need not in all case~ be strictly adhered to.
Hitherto the manufacture of extruded product~ by the hydrolysable unsaturated ~ilane cross-linking technique has been a three-stage process: ~irst the grafted polymer is prepared, secondly it is shaped, ~nd thirdly the shaped article i~ cured. Usually the catalyst is incorporated in the second . .
stage, although it has been suggested that it could be introduced be~ore or during the grafting reaction In accordance with the invention, a method of making a cros~-linked extruded product comprise~: metering into a screw extrusion machine polymer capable of being cross-linked by the use of hydrolysable unsaturated silane together with compounding ingredients comprising a hydrolysable unsaturated silane, a free-radical generator and a silanol condensation 30 ¢atalyst; blending the compounding ingredients with the polymer in the barrel of the said extruder and raising the temperature suf~iciently to effect grafting of silane groups to the polymer, the amount Or ~ree-radical generator being suf~iciently low .: .

4"r. :

- l~J6~10 (havlng regard to the other materials present and to other relevant condltions) to limit direct rree-radical cross-linking to a level that will not prevent extrusion of the material;
e~truding the reaction mi~ture from the said extruder through an e~trusion die to form an elongate ahaped product; and cross-linklng the grafted polymer in the shaped product by the action of moisture. The scrupulous exclusion Or molsture rrom the materials fed to the extruder i8 not required, but, as in conventional e~trusion processes, large amounts of water should not normally be present, and in particular cases a specific drying operation may be necessary. If required, cross-linking can be e~e¢ted at a temperature below the sortening point Or the grafted polymer 90 as to avoid a risk Or distortionO
Other compounding ingredients may be added, for example - antio~ldant~, fillers and pigments.
For processing of olefin polymers, the preferred compounding ingredients are vinyl tri~methoxy silane, a peroxide l that decomposes rapidly at the grafting temperature, and dibutyl i tin dilaurate, but in suitable circumstances any of the ;~ , alternative reagents listed in the said British Specification No. 1286460 may be used. Preferred peroxides are dioumyl pero~ide and 1,3 bis (tertbutyl pero~y iso~ropyl) benzene (sold under the trademark Perkadox 14).
Preferably the polymer and at least some of the compounding ingredients are metered into and premixed in the hopper of the extruder. Suitable apparatus is commercially 1l available, ~or example from Colortronic Reinhard & Co. K - G o~
6382 Friedrichsdorr/Taunus 2, Otto-Hahn-Stras~e 18-20, German Federal Republic, and comprises a hopper with a central powered screw mixer and a number o~ digitally controlled metering devices;
for metering solid materials a rotor having a series of metering chambers filled from above and discharging downwards at a separate station i9 controlled to rotate, usually di~continuously, ~ 3 -~,.. , .. " . ........................ , , ~ ................ , ~:,.,: ~ : : . . . . . . .
. ~. ; , . . . . . . ... .

lO~OlC~
at the required rate, wherea~ for liquid~ an adju~t~ble di~phragm metering pump i8 u~ed.
Ir the number o~ compounding ingredients required makes it nece~3ary, or if desired, ~uitable compsunding ingredient~ may be pre-mixed with the polymer or with each other and metered as a mi~ture; ~or example peroxlde may be fed a~ a ma~terbatch, or a~ a coating on particles of pol~mer or filler;
the conden~atlon catalyst may usually be dissolved in a liquid silane and metered as a solution; and some fillers that are 10 dif~icult to di~perse, notably conductive carbon black, may be better predi~per~d in the polymer or some of it. Provided that sati~factory mixing can be achieved, it may be desirable not to add certain ingredients~ mostly l~quids, to the top of the e~truder hopper, but to in~ect them to the ba~e Or the hopper or direct to the throat of the extruder. ml8 applies in partlcular to ingredients that evolve unpleasant vapour and~or cause the material to stiak to the hopper equipment, notably many of the silanes.
The extruder itself has a single barrel with at least 20 one ~crew ~xtending from end to end of it; u~ually only one screw will be used, but may in~olve sections with di~ferent profile3, po~sibly including intermediate sections without helical flights (a~ for example in two ~tage vented extruders) J and the use of intermeshing screws over part or of all of the length is not excluded. On the other hand the use of two or more ~crews in 3eries is out~ide the scope o~ this application even if they operate in integrally connected barrels, except in the case in which the screw~ operate in the same barrel &nd are directly mechanicall~ coupled to each other, rigidl~ or otherwi3e; in ; 30 this connection attention is drawn to copending Canadian Patent Application ~erlal No. 261178 o~ the Applicant BICC Limited (Michael J. Poole). The e~truder will ordinarily need to provide a premixing zone, a homogenising zone and a metering zone.

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~V~t~V10 Pre~erably th~ hamogenising zone is of the kind, described and claimed in the Applicant Maillefer ~ At~ Brltish Patent No. 964428, in whlch the material to be extruded is rorced over the flight o~ the extruder screw from a converging groove to another groove, not otherwise communicating with the converglng groove, one advantage o~ thi~ arrangement being that entrainment o~ insu~ficiently-softened particulate material i8 inhibited.
When the three zone 8 re~erred to are present, the grR~ting reaction will normally take place in the metering zone.
From the metering zone o~ the extrudsr the homogeni~ed and gra~ted mi~ture passes to the extruder die, which will normally be mounted in a cro~s-head in the case o~ cable manu~acture. me finished product can be cross-linked by exposure to water or a moist atmosphere at a suitable temperature, as in the known three-stage silane gra~ting technique.
` The method of the invention avoid~ the need ~or two - high-temperature processing step~ and for ~toring moisture ~ sensitlve intermediate~ such as gra~ted polymer, whil~t retaining ;; the advantage o~ the silane grarting technique that the e~truded 20 product can be examined ~or dimensional accuracy in a very ~hort period compared with vulcanising and chemical cross-linking methods that involve high-temperature treatment of the e~truded i product under pressure to ef~ect cross-linking. In addition, it has been found that a higher cro~s-link density can be achieved than is normally observed in the ¢onventional three-step hydrolysable unsaturated silane technique; this may perhaps be associated with the elimination of ungrafted polymer ¢onven-tionally used to masterbatch the ca~alyst.
In the ~ollowing examples, wher~e physical properties 30 of extruded materials are ~peci~ied, these are ba~ed on measurements in a typical actual test; the re~ults obtained vary appreciably with the detailed de~ign of the extrusion ~crew.

~ 5 --. .~ ' .

~v~o E~CAMPLE 1 .
Polyethylene with a den3ity Or 0.918 and a melt flow index o~ 2, sold by Imperlal Chemical Industrie~ Ltd. under the trademark Alkathene and re~erence no. WJG47 was fed to the hopper mixer o* an extru~on machine together with the foll~wing metered amountq o~ additive~ (e~pressed in parts by weight per hundred parts o~ the polyethylene (phr):
Carbon black MB 205 Dicum~l peroxide 0.1 (added as Perkadox BC40 which iq 40% active peroxide but calculated on the basis Or the actual peroxide content.) Vinyl trimethoxy silane (V~MOS) 1.5 - Polymerised trimethyl dihydro- 0.5 quinoline (sold under the trademark Flectol H) Dibutyl tin dilaurate (DBDTL) 0.05 The extruder screw had an overall length: diameter ratio of about 30:1 and provided a ~eed zone with a length Or about 8 diameter~ in which the cro~s-~ection of the pas~age 310wly decreases, followed by a homogenising zone of the kind subject of British Patent No. 964428 occupying about 6 diameter~ in 20 whlch, after an initial e~pansion, the material i9 rorced over the flight o~ the ~crew from a rapidly-converging blind pa~sage into a very slowly converging passage. This i~ followed by a ~lightly converging zone of about 6 diameters and finally a metering zone o~ uni~orm cross-~ection occupying the la3t 10 diameter~ o~ the length of the ~crew.
The barrel of the extruder was maintained at a temperature of 130C up to and including the homogeni~ing zones and part of the slightly converging seotion that ~ollow3 it, and at 230C for the remainder of its length, including a cross-head - 30 by whlch the extruded material was applied to a wlre. To ensure good draw-down a higher degree of vacuum iq needed at the point of the cross-head than would be the case for extrus~on o~

1~)6~01(~
ordinar~ thermoplastic pol~ethyleneO
The lnsulation produced had the ~ollowing typical properties arter curing ~or 16 hours in water at 90C:
Gel content 77%
Ultimate tensile strength 13 MN/m2 Elongation at break 270%
Hot-set at 150C and 0.2 M~m2:
exten~ion 35~
set -5%
10 ~XAMPLE 2 -This was similar to Example 1 except that the peroxide wa~ added in 98~ active ~orm and it~ level wa~ reduced to 0.08 phr. Typi¢al properties of the insulation product were:
; Gel content 71~
Ultimate tensile strength 12 MN~mZ
' Elongation at break 350 Hot qet at 150C and 0.2 MN~m2:
e~tension 110%
set ~5%

Example 3 wa~ similar to ~xample 1 except that the peroxide and carbon black were masterbatched with a small amount Or additional polyethylene.
~ ~ample~ 4-7 were ~imilar to Example 3 except that ,; the DBTDL was replaced by the same weight of an alternative , catalyst as follows: Example 4, dibutyltin dimalaate;
E~ample 5, Stanclere T86; Example 6, Stanclere T186; and - E~ample 7, Adva~tab T36 (the word~ Stanclere and Adva~tab being trademark~ and the materials being available on the open market).
Typical properties Or the insulation produ¢ed in these examples were a~ follow~:

, :

- . I
.. ..
, .. , ~ .. ..

0~ ' ~ - ~ -- - ~ - -Example 3 4 5 6 7 ; ~
. ~ . _ _ .~ _.. . . _ . ., ~el content 73 7o 7o 68 69 . __ _ .. . _ - _ . . ____ Ultlm te Ten~ile 16 15 17 14 15 MN/m2 . , . . __ Elongation at Break 380 53 580 380440 _ . _ . _ - . __ ~ot Set: (Elongation 60 7o 95 7o 85 %
150C and ( : 0.2 MN/m2 (Set nil nil nil nil nil %
_ __ __ _ mis was ~imilar to Example 3 except that the V~MOS
content wa~ reduced to 1 phr.

Typical properties of the in~ulation produced were a~
rOllOw~:
Gel content 65%
Ultimate ten~ile ~trength 15 M~/m2 Elongation at break 530%
~ot set: Elongation 110%
Set _5%

These were similar to Example 1 e~cept that the VTMOS
content was raised to 2.0 phr in Examples 10 and 12, and carbon black omitted in Example 12 and the V~MOS content was raised to 205 phr in Example 11, the peroxide in Example 12 being added in -the 96~ active rorm ~old under the trademark Perkadox SB.
. Typical properties Or the insulation produced were as : follows:

~ ~ ~

, . . .
..

106t~010 . ........ .. _ _ Example 10 11 12 - _ :-.~ Gel content 81 79 78 %
Ultimate tensile ~trength 16 11~ 14 MN/m . Elongation at break 350 260 300 %
150C and ( 55 25 35 %
O.Z M~/mZ (~et nil -5 -2.5 '~

. These were similar to Example 12 except that(i) the ;~ 10 pero~ide content was increased to 0015 phr and 0.265 phr respectively and(il) the e~trudate was produced in pipe form and (iii) cured ~or 6 hour9 in bolling water. T~pical properties ,~. .
~ o~ the extruded material produced in theQe e~ample were a~
.~ ~ollows:

, . . .
.. Ex~mple 13 14 , - .. .
. Gel content 83 83 %
. Vicat Penetration 140C 0.54 0.33 m~l .. ~ Ultimate ten~ile strength 1206 12.5 M~/m' .I Elongation at break 280 245 %
Hot set (Elongation 4 32 %
0,2 MN~I (Set nil nil ~
.....

Note: me Vicat test reported in thi~ and subsequent examples is different ~rom the British Standard Recommendation. A load -~1 , : -:~ of lOOg weight was applied by 8 circular ~lat indenter with an . area of 2 mm2 and penetration mea~ured after 10 minutes at '-.~, 140Co ' mi3 was similar to Example 14 except that the !' 30 polyethylene was that sold as Alkathene XNM-68, having a den~ity : _ 9 _ ... . .
.: . ~: : , .
.. ..

106~010 Or 0.924 and a melt-rlow index Or 8, Typical propertie~ of the extruded material wsre a~ follows:
Gel content 79~
Vicat Penetration at 1~0C o.56 mm Yield Stress 11 MN/m2 Ultimate tensile stress 14 MN~m2 Elongation at break 265%

Hot set: (Extension 39%
138C and ( 0.2 MN/m2 (Set nil 10 EXAMP~E 16 Thls wa~ ~milar to Example 15 except that the polyethylene was that sold by Bakelite Xylonite Ltd. as grade PN-220. This is supplied containing carbon black, the denslty of the material as purchased (which should be distinguished ~rom that of the base polymer therein) being 0~930 and its melt ~ flow index 0.12. Typical properties Or the extruded material -l were as follows:
Gel content 79%
Vicat Penetration 140C 0.45 mm Yield stresg 9.6 MN/m2 Ultimate tensile stress 15 M~/m2 Elongation at break 340%

Hot set: (Extension 41%
138C and ( 0.2 M~m2 (Set 2 ¦ Example 17 was similar to Example 16 except that the content of the peroxide wa~ reduced to 0.15 phr. Example 18 was similar to ~xample 17 except that the content of peroxide, ~ilane and catalyst were all reduced by half. Typical properties of the extruded material produced were as follows:
.

-- 10 -- :

106~(~10 . , . _ . , _ .. ~ ':
: Example 17 18 Gel content 74 62 %
Vicat penetratiOn 140C 1.1 107 mm .~ Yield ~tresa 9 10 MN/m2 Ultimate tensile strength 16 18 MN/m2 Elongation at break 44 510 %

138C and (Elngation 63 7o %
O ~2 MN/m2 (Set 2~ _ _ _ _ EXAMPLE 1~
:~ .
Thi~ wa3 similar to Example 11 except that the dicumyl peroxide was replaced by 0 o25 phr o~ 1,3 bi~ (tert. butyl peroxy ~, isopropyl) benzene, added as Perkadox 14~96, which has an activity of about 96% and that the insulation was cured for 6 hours in water at 100C. Typical properties o~ the extruded l material were a~ follows:
.~l Tensile strength 1306 M~m2 ., Elongation at break 215% ~ :
Gel o~ntunt 84%

, i .

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of making a cross-linked extruded product comprising: metering into a screw extrusion machine a polymer capable of being cross-linked by the use of a hydrolysable unsaturated silane together with compounding ingredients comprising a hydrolysable unsaturated silane, a free-radical generator and a silanol condensation catalyst; blending the said compounding ingredients with the said polymer in the barrel of the said extruder and raising the temperature sufficiently to effect the grafting of silane groups to the said polymer, the amount of free-radical generator being sufficiently low to limit direct free-radical cross-linking to a level that will not prevent extrusion of the material; extruding the reaction mixture from the said extrusion machine to form an elongate shaped product; and cross-linking the grafted polymer in the said shaped product by subjecting it to the action of moisture.

2. A method of making a cross-linked extruded product comprising: metering into a screw extrusion machine having in succession a premixing zone, a homogenising zone of the kind in which the material to be extruded is forced over the flight of the extruder screw from a converging groove to another groove not otherwise communicating with the converging groove, and a metering zone a polymer capable of being cross-linked by the use of a hydrolysable unsaturated silane together with compounding ingredients comprising a hydrolysable unsaturated silane, a free-radical generator and a silanol condensation catalyst; blending the said compounding ingredients with the said polymer in the barrel of the said extruder in the mixing and homogenising zones and raising the temperature sufficiently to effect grafting of silane groups to the said polymer in the metering zone, the amount of free-radical generator being sufficiently low to limit direct free-radical cross-linking to to a level that will not prevent extrusion of the material;
extruding the reaction mixture from the said extruder through an extrusion die to form an elongate shaped product; and cross-linking the grafted polymer in the shaped product by subjecting it to the action of moisture.

3. A method as claimed in Claim 1 or Claim 2 in which cross-linking is effected at a temperature below the softening point of the grafted polymer.

4. A method as claimed in Claim 1 or Claim 2 in which the polymer and at least some of the compounding ingredients are metered into and premixed in the hopper of the extruder.

5. A method as claimed in Claim 1 or Claim 2 in which at least one compounding ingredient is metered as a mixture with the polymer or with another compounding ingredient.

6. A method as claimed in Claim 1 or Claim 2 in which the said free-radical generator is a peroxide and is metered as a masterbatch with the polymer or as a coating on particles of polymer or of filler.

7. A method as claimed in Claim 1 or Claim 2 in which a filler is metered as a predispersed mixture with the polymer or same of it.

8. A method as claimed in Claim 1 or Claim 2 in which the said condensation catalyst is metered as a solution in a liquid silane.

9. A method as claimed in Claim 1 or Claim 2 in which at least one liquid compounding ingredient is injected to the base of the hopper of the extruder or injected direct into the throat of the extruder.

10. A method as claimed in Claim 1 in which the said polymer is polyethylene and the said compounding ingredients comprise vinyl trimethoxy silane, a peroxide, and dibutyltin dilaurate.

11. A method as claimed in Claim 10 in which the said peroxide is dicumyl peroxide or 1, 3-bis (tert. butyl peroxy isopropyl) benzene.

12. A method as claimed in Claim 1 or Claim 2 or Claim 10 in which the extrusion machine has a single screw.

13. A method of making electric cable with cross-linked insulation comprising: metering into a screw extrusion machine a polymer capable of being cross-linked by the use of a hydrolysable unsaturated silane together with compounding ingredients comprising a hydrolysable unsaturated silane, a free-radical generator and a silanol condensation catalyst; blending the said compounding ingredients with the said polymer in the barrel of the said extruder and raising the temperature sufficiently to effect the grafting of silane groups to the said polymer, the amount of free-radical generator being sufficiently low to limit direct free-radical cross-linking to a level that will not prevent extrusion of the material; extruding the reaction mixture from the said extrusion machine onto a metallic conductor to form an insulating layer thereon; and cross-linking the grafted polymer in the said insulating layer by subjecting it to the action of moisture.

14. A method as claimed in Claim 13 in which the said polymer is polyethylene and the said compounding ingredients comprise vinyl trimethoxysilane, a peroxide, and dibutyltin dilaurate.