Note: Descriptions are shown in the official language in which they were submitted.
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1 BACKGROUND OF THE INVENTION
2 _
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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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
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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.
18
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
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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.
12
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
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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.
16
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
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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.
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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.
13
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
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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.
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Example 1: (five different blends, all parts by weight)
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6 A B C D E
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
12
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.
21
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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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:
19
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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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:
7 -.
8 Ethylene-propylene rubber100 parts tby weight)
C Polybutene (Vestolen B ~30
10 Vinyltrimethoxisilar.e 1 "
11 Peroxide 0,21 "
12 Catalyst 0,2
13
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.
19
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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