CA1120633A - Moisture crosslinking thermoplastic elastomer - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A thylene-propylene copolymer and/or ethylene vinylacetate copolymer and/or ethylene acrylate copolymer is mixed with propylene, polybutene or polystyrol as well as silane which is grafted onto the copolymer at around200°c and causes subsequent crosslinking in any available moisture, ambient or cooling water.
Crosslinking follows the extrusion of the blend around a conductor, a tube or the like.

Description

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3 The present invention relates to a rubber-like thermoplastic material which thermo-sets or crosslinks in the presence of moisture.
7 Thermoplastic material of the type to which the invention 8 pertains is to be used for example as coating or jacket of 9 elongated stock such as electrical conductors, cables, tubes or the like. It is well known in the art that rubber-like hydrocarbons 11 such as synthetic or natural rubber, i.e. elastomeric material can 12 be crosslinked (vulcanized) under the action of a suitable~free 13 radical generator such as an organic peroxide. This material 14¦ crosslinks in situ, on elonsated stock to which it has been applied 15¦ previously by stringing the coated stock through a tubular chamber 16 ¦ for continuous vulcanization. These tubes are filled with hot, 17¦ pressurized steam. In order to obtain particular mechanical 181 properties as they are required~ e.g. for cable or conductors or 19¦ for hoses, one strengthens the elastomer by means of organic fillers 20¦ such as carbon black, silicic acid, Kaolin, etc.
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22¦ The known methods of crosslinking these rubber-like hydro-23¦ carbons require extensive equipment (capital investment) which 241 occupy a considerable amount of space in a manufacturing facility.
251 Still, the known equipment exhibits a considerable interdependence - 26¦ of pressure and temperature as they cannot be adjusted independently.

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~ ,(?fi33 _, 1 Thus it was found that a complex multi-conductor cable was unduly 2 compressed by the high pressure oE the saturated steam, having , 3 the requisite vulcanizing temperature. Even minor damage to an insulation coating on a conductor may result in penetration of water so that the coating or jacket may lose locally its insulative 6 property. Subsequent drying for removal of the water from the 7 interior of the jacket was found to diminish the mechanical proper- i 8 ties, i.e. strength of the cable.

The known crosslinking methods are further disadvantaged in some 11 instances by the direct coupling of the vulcanization tube and the 12 extruder head which applies the jacket to the stock. The plastic 13 may be scorched as it is applied. Stock of large cross-section 14 may more readily contact the interior surface of the vulcanization tube and the coating may be scratched. This damage is even more 16 pronounced if the interior of the tubular vulcanizing chamber begins 17 to corrode. The various drawbacks outlined above have lead to 1~ the development of alternatives for the vulcanization equipment.
19 For example, instead of steam one has used ultrahigh frequencies;
saline baths, fluidized beds and thermal radiation. However, in 21 spite of considerable efforts along these lines, none of these 22 methods has been able to successfully replace hot steam vulcani-2~ zation, at least not for mass production purposes. This is primarily 24 due to the fact that the heat transfer or heat development necessary ' ~, 2r~ for activating the crosslinking reaction is quite complicated in all 26 these alternative attempts. Moreover, these alternative methods 27 .

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1~ ~?Ji~633 1 operate basically without the application of pressure. Thus, so-2 called vacuum or vent type extruders are needed in order to avoid 3 the formation of any porosity in the jacket or coating.
5 Further, as far as the state of the art is concerned, the 6 mixing of the raw material is usually carried out with conventional 7 means of long standing such as mixers or a rolling mill. The 8 polymer, plasticizer, anorganic fillers and other additives are 9 homogenized basical]y under utilization of shear forces being f 10 mechanically applied. A second step adds the crosslinking agents 11 and vulcanization accellerators rather carefully, particularly 12 as far as temperature is concerned. The extruder plasticizes the 13 material and shapes it into the desired configuration. In the case 14 of low pressure crosslinking all components of lower molecu'ar 15 weight as well as water have to be removed from the mixture.
16 L. Werwitzke has reported on the state of the art in "Draht" vol 19 17 (1968) No 8, pages 593 et seq.

19 Recently, another line of development has lead to the cross- ¦
20 linking of thermoplastic material such as polyethylene in the 21 presence of moisture, whereby a silicon compound or a mixture of 22 such components has been grafted onto the base molecules. This 2~ method can qulte successfully be used for jacketing cables with 24 thermo-set polyethylene. See for example US patents 3,646,155;
25 4,Q58,583; or German printed application 2,411,141.

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DESCRIPTION OF THE INVENTION
The present invention seeks to provide a thermoplastic elastomer which can crosslink in the presence of moisture without requiring extensive vulcanizing equipment.
In accordance with the preferred embodiment of the present invention, it is suggested to use a blend which includes, as a first component, a rubber like copolymer of ethylene such as ethylenepropylene copolymer, ethylene-propylene terpolymer rubber or others (infra); crosslinkable molecules have been grafted onto the molecules of the copolymer and/or added to double bonds f the copolymer to obtain later crosslinking to the presence of moisture, the blend includes as a second component a partially crystalline or amorphous polvolefin with tertiary or quarternary hydrocarbon atoms, such as isotactic and syndiotactic polypropylene which will not tend to crosslink by operation of the respective radicals but serve only as an organic filler.
It was found that such a material when conventionally applied to stock by means of a vacuum extruder differs little from a rubber base coating as far as outer appearance is concerned. If compared with the known moisture crosslinking thermoplastic (polyethylene) requiring silane grafting, one needs considerably lower grafting temperature, 150 vs 200 to 240C. Also, the residence time in the barrel for grafting is-~much shorter, even if considering ~o~

1 the temperature dependency of such a residence time. A major 2 ¦ advantage of the invention over the fluidized bed vulcanization, 3 saline bath crosslinking etc. is to be seen in the fact that post-extrusion treatment of the coating to obtain crosslinking can be dispensed with entirely. The grafted material as per the present 6 invention crosslinks right after shaping (i.e. right after having 7 been applied as a coating) and cooiing,or at least shortly there-8 after. Thus, one does not have to run the coated stock into or 9 through a very long water container for purposes of applying moisture thereto. Rather, pulling the stock through a tank with cooling 11 water suffices for applying the requisite moisture.

13 In order to obtain moisture crosslinking of the blend in ld accordance with the invention one may use graftable silane comporlents such as a mixture of silane and peroxide.(see e.g. German printed 16 patent application 1,794,028 or US patent 3,646,155; 4,058,583).
17 Alternatively, however, one does not need peroxide or other free 18 radical generatOrs for ini.iating grafting, provided the polymer 19 chain or the base material (first component, supra) has available a sufficient quantity of double bonds which may serve as reactive 21 centers for adding silane hydrides. Components to be used for addiny j 22 are those whose molecules contain the silicon hydride group 23 ~Sl -H at least once.

Preferred rubber-l]`ke copolymers to be used as the first 27 component in the blend as per the present invention are ethylene , I

propylene copolymers such as ethylene propylene rubber or ethylene 2 propylene terpolymer rubber; mentioned above; other copolymers to 3 be used are ethylene-vinyl acetate copolymer and ethylene acry~ate copolymer. These copolymers can be used by themselves or in a blend still constituting the first component. Organic and anorganic fillers 6 could be added for obtaining particular properties of the resulting 7 product. For example, aerosil (SiO2) can be added at a certain rate 8 to obtain a particu]ar surface roughness of the product. Of course, 9 this filler should not be used extensively. Thus, one uses 0.1 to parts by weight per 100 parts copolymer. TiO2 can be added as 11 a pigment, either by itself or in addition to the additive above, to 12 brighten the color of the product; carbon black can be added to 13 weatherproof the cable, i.e. stabilize it against ultraviolet 14 radiation. All these additives together should not exceed about 0-5 to 10parts by weight per 100 parts copolymer.

17 As far as the partially crystalline polyolefins (component #2) 18 are concerned, they are to be added as organic fillers without parti-19 cipating in the crosslinking, directly or indirectly, in that silane or other material that is grafted on the copolymer (component ~1) wil 21 not be grafted onto this component ~2. One will preferably use iso- ~
22 tactic polybutene or polypropylene. Another suitable polyolefin which 23 does not participate in crosslinking should be of the amorphous type 2~ with a high congealing temperature such as polystyrol. Depending on the two basic polymer components used,one can adjust the flexibility-26 and heat resistance of the resulting product. For example, 70 parts27 copo]ymer and 3Q parts (by weight) polypropylene is a very suitable ratio for cable. In order to o~tain any significant effect, this ~, 112~633 ~

1 ¦ second component should be at least 10 parts by weight for 100 parts ; 2 ¦ by weight copolymer. However, this non-graftible, non-crosslinking ¦ second component should be present at a smaller amount than t~e

4 ¦ copolymer, but can be added in certain cases up to 200 parts per ¦ 100 parts crosslinkable copolymer.
¦ The particular composition is preferably made as follows. First, 7 ¦ the particular additivesneeded for crosslinking, i.e. the silane or 8 ¦ a mixture of silane is added to a blend of ethylene-propylene rubber, ¦ and polypropylene. The latter blend is actually available as a ff~ C 10 ¦ product under the designation Dutral TP30~ and constitutes a mixture 11¦ of the two major components. This mixture is mixed with the silane 12 ¦ in a tumble mixer or any other kind of high speed mixer. The silane 13¦ will diffuse into the porous rubber and even a slight increase in 141 temperature (20 to 40C) speeds up the diffusion process considerably 15¦ so that the silane is rather homogeneously distributed in the co-16 ¦ polymer and polypropylene blend which is still a powder or granu-17¦ late.
181 1, 19¦ Next, the silane is grafted onto the elastomeric copolymcr 20 ¦ molecules. The polypropylene does not participate in the grafting.
21 ¦ One can proceed now in two ways. In one mode of operation grafting 22¦ is carried out in an extruder which may actually provide the final ~ f 2~ ¦ shape of the product, i.e. it may extrude the blend onto and around 2~f ¦ a wire, a tube, etc. while grafting takes place. On the other hand 251 grafting and granulation could be combined and the shape or form .~6 ¦ extrusion will he carried out subsequently.
2~7 1 ~ r~e ~ rk 2~31 _ ' llZl~fi33 -' 1 The following procedure was found to be the preferred mode of ; , 2 practicing the invention. The graftible thermoplastic elastomer 3 and copolymer, e.g~an ethylene-propylene copolymer and the non-graftible filler, e.g. polypropylene are charged in a tumble mixer

5 ¦ or a high speed mixer and the additives needed for crosslinking

6 ¦ are added thereto, and all the aharges are blended. Mixing occurs at room temperature and the blend becomes quite homogenic. Par.ticu-8 larly the crosslinking additives are distributed to be located where 9 needed; the rubber portion of the blend has a high af-finity to the silane. Thus,favorable distribution of the silane enhances the 11 degree of crosslinking which can be attained later. Mixing is quite 12 short, i.e. shorter than in the case of polyethylene alone.

14 The polyblen~ as produced permits grafting at temperatures between 150 and 200CC which is considerably below the grafting 16 temperature of polyethylene. The graftible elastomeric compound 17 (copolymer) and the partially crystalline polypropylene are homo-18 genically mixed by the shear forces in the extruder while grafting 19 occurs at a temperature around 200C requiring less than 1 minute.
If the coating or jacket is made on a cable or conductor which 21 is to serve for the transmission of medium or high voltages, 22 particularly high requirements as to insulative strength have to 23 be met and the homogeneity of the components in the blend is 24 particularly critical. Accordingly, one should use here an ex-truder with a venting zone for degassing in order to avoid the formation . ~ .
26 of bubbles in the extrudcd coating.

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j 1 The following examples demonstrate with particularity how the l -: 2 invention can be practiced.
: 3 Example 1: (five different blends, all parts by weight) ~ _ I

71 Ethylene-propylene 8 ¦ Copolymer 100 100 100 100 100 I Polypropylene 15 25 40 50 . 60 10 ¦ Vinyltrimethoxisilane 11¦ Peroxide 0,21 0,21 0,21 0,210,21 I Catalyst 0,05 0,05 0,05 0,050,05 13 ¦ Age Protection 0,2 0,2 0,2 0,2 0,2 14 ¦ The peroxides that can be used are e.g. those traded under the C¦ designation Perkadox 14/40 and Luperox 270~which are 1,3 (Bis-tert.-16 ¦ butylperoxi-isopropyl) Benzol and tert. Butyl-isonanoane.
17 ¦ T-he catalyst may be dibutyltindilaurate (e.g. as traded under the 18 ¦ designation Naftovin SN/~ . Age protection may be Anox HB which is 29 ¦ 2,2,4-Trimethyl- 1,2,-dihydroquinoline. The various examples dlffer only by the amount of non-graftible additive, polypropylene.

22 The several components are mixed for up to about four minutes.
23 Subsequently, one will graft the silane as described. The final 24 products crosslinked at, beginning with composition A, between 66 and 40 gel percentages.
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~,7 ,,,,. ~ -1~633 o l 1 Example 2: (Three different blends, all parts by weight) 3 F : G H
4 Ethylene-propylene Copolymer 100 100 100 6 Polypropylene 30 30 30

7 Vinyltrimethoxisilane 0,5 ,0,75 1,0 Peroxide ) 0,210,21 0,21

8 ) supra 0,050,05 0,05 Age protection 0,2 0,2 0,2 11 Upon comparing example 1 with example 2, one can see that in 13 example 2 the rubber polypropylene ratio remained the same, but the silane content was increased. .~ixing lasted also about four minutes l_ and grafting was induced at a temperature below 200C but the degree 16 of crosslinking was increased from about 50 gel ~ for F, to 17 about 60gel% for H.
18 Example 3:

20 Ethylene propylene rubber 100 parts (by weight) 21 Polyethylene~vinylacetate 30 " " I
22 (Baylon V18H~ j 23 Vinyltrimethoxisilane 1 " "
24 Peroxide ) 0.21 "
251 Catalyst ) 0.05 " "

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~ 33 1 Only a similarly short mixing period of a few minutes was 2 required e.g. in a tumble mixer, and the grafting temperature was also below 200C. One obtained a degree of grafting of over ~ 80 gel %.

6 Example 4:
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8 Ethylene-propylene rubber100 parts tby weight) C Polybutene (Vestolen B ~30 10 Vinyltrimethoxisilar.e 1 "
11 Peroxide 0,21 "
12 Catalyst 0,2 14 Again, not more than four minutes was required and the yrafting temperature was below 200C for a degree of crosslinking of about 16 60 gel %. The residence time for grafting is about 30 to 60 17 seconds from a temperature of about 150C, but 10 to 20 seconds 18 suffice for 180 grafting temperature.

The invention is not lîmited to the embodiments described 21 above but all changes and modifications thereof not constituting 22 departures from the spirit and scope of the invçntion are intended 23 to be included.
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Claims (10)

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

1. Rubber-like thermoplastic base material for use as coating or jacket comprising a blend of rubber-like copolymer of ethylene with a partially crystalline or amorphous polyolefin having tertiary or quarternary hydrocarbon atoms and being incapable of cross-linking by operation of radicals, silane molecules being attached to said copolymer of the ethylene molecules, said silane molecules being capable of cross-linking the copolymer in the presence of moisture.

2. The material as in claim 1, wherein the said silane molecule contains a group at least once.

3. The material as in claim 1, said copolymer being one or a blend of the following: an ethylene propylene rubber, an ethylene propylene ter-polymer rubber, an ethylene vinylacetate copolymer or an ethylene acrylic acid ester copolymer.

4. A material as in claim 1, said partially crystalline polyolefin being an isotactic polybutene or polypropylene or amorphous polyolefin with high congealing temperature.

5. A material as in claim 1, using an amorphous polyolefin with a high congealing temperature.

6. A material as in claim 5, said polyolefin being polystyrol.

7. A method of providing a cross-linked elastomer, comprising the steps of mixing a rubber-like copolymer of ethylene with a partially crystal-line polyolefin which does not cross-link;

adding thereto silane or a silane compound and other additives needed for grafting;
grafting the silane onto the copolymer;
shaping the mixture after the grafting.

8. A method as in Claim 7, wherein grafting and shaping is carried out in one continuous step.

9. A method as in Claim 7 or 8, wherein shaping is carried out by extrusion.

10. A method as in Claim 7 or 8, the shaping step being extruding the mixture onto and around elongated stock.

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