U.S. patent application number 15/106227 was filed with the patent office on 2016-10-27 for semiconductive polymer composition for electric power cables.
The applicant listed for this patent is BOREALIS AG. Invention is credited to Karl-Michael JAGER, Malin JOHANSSON, Koenraad NOYENS, Fredrik SKOGMAN, Christer SVANBERG, Takashi UEMATSU, Peter WALTER.
Application Number | 20160311998 15/106227 |
Document ID | / |
Family ID | 50002396 |
Filed Date | 2016-10-27 |
United States Patent
Application |
20160311998 |
Kind Code |
A1 |
UEMATSU; Takashi ; et
al. |
October 27, 2016 |
SEMICONDUCTIVE POLYMER COMPOSITION FOR ELECTRIC POWER CABLES
Abstract
The invention provides a novel semiconductive polymer
composition with improved smoothness and dispersibility of carbon
black when compounding the polymer composition and feasible balance
with other properties such as volume resistivity. The
semiconductive polymer composition comprises (a) from 30 to 90 wt %
of a polymer component, (b) from 10 to 70 wt % of carbon black and
the carbon black (b) has a mass pellet strength (MPS) according to
ASTM D1937-13 of from 50 to 250 N. The invention further relates to
a process for preparing the semiconductive polymer composition
comprising the steps of: i) introducing 30-90 wt % of a polymer
component as defined above and 0-8 wt % additives in a mixer device
and mixing the polymer component and additives at elevated
temperature such that a polymer melt is obtained; ii) adding 10-70
wt % of a carbon black as defined above to the polymer melt and
further mixing of the polymer melt.
Inventors: |
UEMATSU; Takashi;
(Stenungsund, SE) ; SVANBERG; Christer; (Kallered,
SE) ; JAGER; Karl-Michael; (Goteborg, SE) ;
SKOGMAN; Fredrik; (Stenungsund, SE) ; NOYENS;
Koenraad; (Geel, BE) ; WALTER; Peter;
(Savedalen, SE) ; JOHANSSON; Malin; (Stenungsund,
SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BOREALIS AG |
Vienna |
|
AT |
|
|
Family ID: |
50002396 |
Appl. No.: |
15/106227 |
Filed: |
December 18, 2014 |
PCT Filed: |
December 18, 2014 |
PCT NO: |
PCT/EP2014/003420 |
371 Date: |
June 17, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B29K 2507/04 20130101;
B29K 2023/0633 20130101; H01B 7/0216 20130101; C08K 3/04 20130101;
B29B 9/10 20130101; C08K 3/04 20130101; B29K 2105/16 20130101; C08L
2203/202 20130101; B29K 2995/0005 20130101; B29B 9/12 20130101;
C08L 23/0846 20130101 |
International
Class: |
C08K 3/04 20060101
C08K003/04; B29B 9/12 20060101 B29B009/12; B29B 9/10 20060101
B29B009/10; H01B 7/02 20060101 H01B007/02 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 20, 2013 |
EP |
13005982.7 |
Claims
1-20. (canceled)
21. A semiconductive polymer composition comprising (a) from 30 to
90 wt % of a polymer component, (b) from 10 to 70 wt % of carbon
black and wherein the carbon black (b) has a mass pellet strength
(MPS) according to ASTM D1937-13 of from 50 to 250 N.
22. The semiconductive polymer composition according to claim 21,
wherein the carbon black (b) has an average value of individual
pellet hardness (CSAV) according to ASTM D5230-13 of from 5 to 30
cN.
23. The semiconductive polymer composition according to claim 21,
wherein the carbon black (b) has an average value of individual
pellet hardness for the 5 hardest pellets (M5H) according to ASTM
D5230-13 of from 10 to 40 cN.
24. The semiconductive polymer composition according to claim 21,
wherein the carbon black (b) has an average pellet size according
to ASTM D1511-12 of from 0.1 to 5 mm.
25. The semiconductive polymer composition according to claim 21
wherein the carbon black (b) has one or more of the following
characteristics: a BET surface area (STSA value), measured by
nitrogen adsorption according to ASTM D 6556-10 of from 20 to 60
m.sup.2/g; an iodine adsorption number measured according to ASTM
D1510-13, method A of from 20 to 60 g/kg; a DBP oil absorption
number measured according to ASTM D2414-13 of from 100 to 150
cm.sup.3/100 g.
26. The semiconductive polymer composition according to claim 21
wherein the composition has a surface smoothness measured according
to the surface smoothness analysis using a tape sample as described
herein of not more than 200 particles/m.sup.2 having a width of
larger than 150 .mu.m, and/or not more than 9 particles/m.sup.2
having a width of larger than 200 .mu.m.
27. The semiconductive polymer composition according to claim 21
wherein said carbon black (b) is furnace carbon black.
28. The semiconductive polymer composition according to claim 21,
wherein said polymer component (a) comprises an alpha-olefin
polymer.
29. The semiconductive polymer composition according to claim 21,
wherein said polymer component (a) comprises a homopolymer of a
C.sub.2-12 alpha-olefin or a copolymer of a C.sub.2-8 alpha-olefin
with one or more comonomers of an C.sub.3-30 alpha-olefin.
30. The semiconductive polymer composition according to claim 21,
wherein polymer component (a) is selected from a branched ethylene
homo- or copolymer and a linear ethylene homo- or copolymer.
31. The semiconductive polymer composition according to claim 21,
wherein said polymer component (a) comprises at least one
polyunsaturated comonomer.
32. The semiconductive polymer composition according to claim 21,
wherein said polymer component (a) comprises at least one polar
comonomer.
33. The semiconductive polymer composition according to claim 32,
wherein said polar comonomer is selected from: vinyl carboxylate
esters, such as vinyl acetate and vinyl pivalate, (meth)acrylates,
such as methyl(meth)acrylate, ethyl(meth)acrylate,
butyl(meth)acrylate and hydroxyethyl(meth)acrylate, olefinically
unsaturated carboxylic acids, such as (meth)acrylic acid, maleic
acid and fumaric acid, (meth)acrylic acid derivatives, such as
(meth)acrylonitrile and (meth)acrylic amide, vinyl ethers, such as
vinyl methyl ether and vinyl phenyl ether.
34. The semiconductive polymer composition according to claim 32
wherein the content of polar comonomer in said polymer component
(a) is from 0.5 to 35 wt %, based on the total amount of said
polymer component (a).
35. The semiconductive polymer composition according to claim 21
which has a MFR.sub.21, of from 1.0 g/10 min to 15 g/10 min,
measured according to ISO 1133 at 125.degree. C. and a load of 21.6
kg.
36. The semiconductive polymer composition according to claim 32,
which is cross-linkable via radical reaction or via silane
groups.
37. A process for preparing a semiconductive polymer composition
comprising the steps of: i) introducing 30-90 wt % of the polymer
component (a) as defined in claims 21 and 0-8 wt % additives in a
mixer device and mixing the polymer component and additives at
elevated temperature such that a polymer melt is obtained; ii)
adding 10-70 wt % of a carbon black as defined in claim 21 and
further mixing of the polymer melt to obtain a semiconductive
polymer mixture, iii) extruding and pelletising the obtained
polymer mixture.
38. An electric power cable comprising a conductor surrounded by
one or more layers, wherein at least one of said one or more layers
is a semiconductive layer, which comprises a semiconductive polymer
composition as defined in claim 21.
39. The electric power cable according to claim 38, wherein at
least one of the inner and outer semiconductive layer(s) comprises
a semiconductive polymer composition as defined in claim 21.
Description
[0001] The present invention relates to a semiconductive polymer
composition, to a method for preparing said semiconductive polymer
composition, to its use for the production of a semiconductive
layer of an electric power cable, and to an electric power cable
comprising at least one semiconductive layer, which layer comprises
the above mentioned semiconductive polymer composition.
[0002] In wire and cable applications a typical cable comprises at
least one conductor surrounded by one or more layers of polymeric
materials. In power cables, including medium voltage (MV), high
voltage (HV) and extra high voltage (EHV), said conductor is
surrounded by several layers including an inner semiconductive
layer, an insulation layer and an outer semiconductive layer, in
that order. The cables are commonly produced by extruding the
layers on a conductor. Such polymeric semiconductive layers are
well known and widely used in dielectric power cables rated for
voltages greater than 1 kilo Volt. These layers are used to provide
layers of intermediate resistivity between the conductor and the
insulation, and between the insulation and the ground or neutral
potential.
[0003] These compositions are usually prepared in granular or
pellet form. Polyolefin formulations such as these are disclosed in
U.S. Pat. Nos. 4,286,023; 4,612,139; and 5,556,697; and European
Patent 420 271. One or more of said layers of the power cable are
typically crosslinked to achieve desired properties to the end
product cable. Crosslinking of polymers, i.e. forming primarily
interpolymer crosslinks (bridges), is one well known modification
method in many end applications of polymers. Crosslinking of
polymers, such as polyolefins, substantially contributes i.a. to
heat and deformation resistance, creep properties, mechanical
strength, as well as to chemical and abrasion resistance of a
polymer. In wire and cable applications crosslinked polymers, such
as crosslinked polyethylenes, are commonly used as a layer
material, e.g. in insulating, semi-conducting and/or jacketing
layers.
[0004] The purpose of a semiconductive layer is to prolong the
service life, i.e. long term viability, of a power cable i.a. by
preventing partial discharge at the interface of conductive and
dielectric layers. Surface smoothness of the extruded
semiconductive layer is a property that plays an important role in
prolonging the surface life of the cable. The smoothness is
influenced i.a. by the type of the used carbon black. An uneven
distribution of the particle size of carbon black particles can
adversely affect said surface smoothness and cause localised
electrical stress concentration which is a defect that can initiate
a phenomenon well known as vented trees. Moreover, the surface
properties and particle size as such of the carbon black may affect
the surface smoothness of the semiconductive layer of a power
cable. For example, it is known that the larger the carbon black
particles are, the smoother the surface of the semiconductive layer
is.
[0005] However, increasing the particle size of a carbon black for
improving smoothness in turn deteriorates, i.e. increases, the
resistivity of the semiconductive layer material, whereby these
properties need often be balanced. Especially in case of so called
furnace carbon black the contradicting properties of surface
smoothness and volume resistivity are pronounced.
[0006] Furnace carbon black is a generally acknowledged term for
the well known carbon black type that is produced in a furnace-type
reactor by pyrolyzing a hydrocarbon feedstock with hot combustion
gases. A variety of preparation methods thereof are known and such
furnace carbon blacks are described i.a. in EP 0 629 222, U.S. Pat.
No. 4,391,789, U.S. Pat. No. 3,922,335, U.S. Pat. No. 3,401,020 and
U.S. Pat. No. 6,086,792. Furnace carbon black is distinguished
herein from acetylene carbon black which is also a generally
acknowledged term for the well known type of carbon black produced
by reaction of acetylene and unsaturated hydrocarbons, e.g. as
described in U.S. Pat. No. 4,340,577.
[0007] Commercial furnace carbon black grades are e.g. described in
ASTM D 1765-13 such as N351, N293 and N550. Carbon black N550 is
described e.g. in US 2011/0186328 A1. The letter "N" indicates a
normal curing rate of a typical rubber compound containing the
carbon black. It is typical for furnace carbon black that has not
received a special modification in order to alter its influence on
the rate of cure of rubber. However, furnace carbon black is
usually less suited for semiconductive compositions used in wire
and cable applications due to its rather high volume resistivity
and poor dispersion properties in polymers.
[0008] WO 2009/053042 A1 relates to a semiconductive polymer
composition for the production of a semiconductive layer of an
electric power cable, which exhibits improved surface smoothness
and has good balance with other properties needed for a
semiconductive polymer material. The semiconductive polymer
composition comprises a polyolefin component, carbon black and
additives. The carbon black used for preparing the semiconductive
composition was a commercially available furnace carbon black.
[0009] Moreover, many carbon blacks, e.g. the above mentioned
furnace carbon blacks, are commercially available in a form of
"pellet" agglomerates formed from primary carbon black particles
thereof. These agglomerates are broken during the processing, i.e.
during compounding in the preparation of said semiconductive
polymer composition. The breakdown of said agglomerates thus may
also have an effect on said surface smoothness property. Without
binding to any theory it appears that an extensive mixing of
semiconductive polymer mixture in order to get an even particle
size distribution amongst the carbon black particles may adversely
affect the resistivity of the composition. Accordingly there seem
to be limitations in the particle size window for the carbon black
particles to enable sufficient smoothness and resistivity of the
final product.
[0010] Thus, it is the object underlying the present invention to
provide a novel semiconductive polymer composition with improved
smoothness and dispersibility of carbon black when compounding the
polymer composition and at the same time feasible balance with
other properties such as volume resistivity.
[0011] The inventors found that this object is achieved by a
semiconductive polymer composition comprising:
(a) from 30 to 90 wt % of a polymer component, (b) from 10 to 70 wt
% of carbon black and wherein the carbon black (b) has a mass
pellet strength (MPS) according to ASTM D1937-13 of from 50 to 250
N, preferably from 50 to 200 N, even more preferably from 50 to 180
N.
[0012] Especially the invention is based on the finding that the
modification of the pellet crush resistance of a conventional
carbon black (medium to high pellet crush resistance) gives an
excellent surface smoothness for a semiconductive polymer
composition even if the carbon black content of the composition is
rather high, e.g. 20 wt % or more, 30 wt % or more, or even 40 wt %
or more, based on the total weight semiconductive polymer
composition. Still the volume resistivity is excellent. For the
purposes of the present invention, a volume resistivity of 500 ohmm
is beneficial. Thus, a compromise between the contradicting
properties of surface smoothness and volume resistivity of a
semiconductive polymer composition not reached before, could be
achieved.
[0013] Preferably, in the semiconductive polymer composition of the
invention the carbon black (b) has an average value of individual
pellet hardness (CSAV) according to ASTM D5230-13 of from 5 to 30
cN, more preferably from 10 to 30 cN.
[0014] The carbon black (b) may further have an average value of
individual pellet hardness for the 5 hardest pellets (M5H)
according to ASTM D5230-13 of from 10 to 40 cN, preferably from 15
to 35 cN.
[0015] The properties of mass pellet strength according to ASTM
D1937-13 and individual pellet hardness according to ASTM D5230-13
reflect the toughness of the carbon black pellets and the
above-defined ranges surprisingly give an excellent dispersion of
the carbon black during compounding and extrusion. Individual
pellet hardness encompasses the parameters of CSAV and M5H as
defined under "Determination Methods". If the parameters of mass
pellet strength and individual pellet hardness are above the
defined ranges, the crushing of the pellets is difficult, resulting
in poorer carbon black dispersion or a roughened surface of the
composition and tape layer. If the above parameters are below the
defined ranges, the pellets result in an increased level of fines
and processability of the semiconductive polymer composition will
decrease.
[0016] The carbon black (b) may preferably have an average pellet
size according to ASTM D1511-12 of from 0.1 to 5 mm, more
preferably from 0.5 to 3 mm, even more preferably from 0.6 to 2 mm.
Any of the above lower limits may be combined with any of the above
upper limits to form another preferred range for the average pellet
size of the carbon black.
[0017] According to further preferred embodiments the carbon black
(b) may have one or more of the following characteristics:
a BET surface area (STSA value), measured by nitrogen adsorption
according to ASTM D 6556-10 of from 20 to 60 m.sup.2/g, preferably
from 30 to 50 m.sup.2/g, more preferably from 35 to 45 m.sup.2/g;
an iodine adsorption number measured according to ASTM D1510-13,
method A of from 20 to 60 g/kg, preferably from 30 to 55 g/kg, more
preferably from 38 to 48 g/kg; a DBP oil absorption number measured
according to ASTM D2414-13 of from 100 to 150 cm.sup.3/100 g,
preferably from 110 to 140 cm.sup.3/100 g, more preferably from 116
to 126 cm.sup.3/100 g.
[0018] The semiconductive polymer composition preferably has a
surface smoothness measured in accordance with the Surface
Smoothness Analysis (SSA) method as defined under "Determination
Methods" using a tape sample as described below of not more than
200 particles/m.sup.2, preferably not more than 100
particles/m.sup.2, even more preferably not more than 50
particles/m.sup.2 having a width of larger than 150 .mu.m, and not
more than 9 particles/m.sup.2, preferably not more than 8
particles/m.sup.2, even more preferably not more than 6
particles/m.sup.2 having a width of larger than 200 .mu.m.
[0019] As the polymer component (a), any commercial polymer or a
polymer obtainable by a commercial polymerization process can be
used in the polymer composition of the invention. The amount of
said polymer component (a) of said semiconductive polymer
composition of the invention is preferably of from 40 to 75 wt %,
more preferably of from 50 to 70 wt %. Said polymer component (a)
of said semiconductive polymer composition is preferably a
polyolefin, more preferably a polymer of an alpha-olefin which may
include a homopolymer of ethylene or copolymer of ethylene with one
or more comonomers, which is selected from a branched polyethylene
homo- or copolymer produced at high pressure by free radical
initiated polymerisation (referred to as high pressure radical
polymerization) and well known as low density polyethylene (LDPE)
homopolymer, or copolymer, which is referred herein as LDPE homo-
or copolymer, or a linear polyethylene homo- or copolymer produced
by low pressure polymerisation using a coordination catalyst, such
as well known linear very low density polyethylene (VLDPE), linear
low density polyethylene (LLDPE), medium density polyethylene
(MDPE) or high density polyethylene (HDPE), which is referred
herein as "linear PE homo- or copolymer", or a mixture of such
polymers.
[0020] The polyethylene as defined above suitable as said polymer
component (a) can be said linear PE homo- or copolymer, which is
preferably VLDPE, LLDPE, MDPE or HDPE polymer. They can be produced
in a known manner in a single or multistage processes e.g. as
slurry polymerisation, a solution polymerisation, a gas phase
polymerisation, and in case of multistage process in any
combination(s) thereof, in any order, using one or more of e.g.
Ziegler-Natta catalysts, single site catalysts, including
metallocenes and non-metallocenes, and Cr-catalysts. The
preparation of linear ethylene polymer is and the used catalysts
are very well known in the field, and as an example only, reference
is made i.a. to a multistage process described in EP517868.
[0021] The preferred polymer component (a) of the invention is said
polyethylene as defined above and is more preferably said LDPE homo
or copolymer which may optionally have an unsaturation that can
preferably be provided by copolymerising ethylene with at least one
polyunsaturated comonomer as defined above and/or by using a chain
transfer agent, such as propylene. Such polymers are well known and
described e.g. in WO 93/08222, EP 1695996 or WO2006/131266.
Typically said unsaturated polyolefins have a double bond content
of more than 0.1 double bonds/1000 C-atoms.
[0022] Further, the LDPE homo or copolymer subgroup of said
preferred ethylene polymers is more preferably an LDPE copolymer of
ethylene with one or more comonomers which are preferably selected
from:
C.sub.3 or higher olefin comonomer(s), preferably
(C.sub.3-C.sub.30)alpha-olefin comonomer(s), more preferably
(C.sub.3-C.sub.12)alpha-olefin comonomer(s); polar comonomer(s),
silane comonomer(s) or polyunsaturated comonomer(s), e.g. a
comonomer with at least two double bonds, such as diene comonomers;
or a mixture of the above mentioned comonomers, and which LDPE
copolymer may optionally have a further unsaturation provided by
using a chain transfer agent, such as propylene, and which LDPE
copolymer is referred herein as LDPE copolymer. The comonomers
mentioned above, as well as chain transfer agents are well known in
the art. The polar groups of said polar comonomer are preferably
selected from siloxane, amide, anhydride, carboxylic, carbonyl,
hydroxyl, ester and epoxy groups.
[0023] Most preferred polymer component (a) of said semiconductive
polymer composition is said LDPE copolymer, more preferably is a
LDPE copolymer, wherein the comonomer is selected from one or more
of polar comonomer(s) and may optionally comprise an unsaturation
provided preferably by copolymerising ethylene with at least one
polyunsaturated comonomer(s) and/or by using a chain transfer
agent, such as propylene, as defined above, which LDPE copolymer is
referred herein as LDPE copolymer of ethylene with at least polar
comonomer(s), and is most preferably an LDPE copolymer of ethylene
and at least polar comonomer(s). More preferably, said polar
comonomer(s) in said LDPE copolymer of ethylene with at least polar
comonomer(s) for said semiconductive polymer composition is/are
selected from: vinyl carboxylate esters, such as vinyl acetate and
vinyl pivalate, (meth)acrylates, such as methyl(meth)acrylate,
ethyl(meth)acrylate, butyl(meth)-acrylate and
hydroxyethyl(meth)acrylate, olefinically unsaturated carboxylic
acids, such as (meth)acrylic acid, maleic acid and fumaric acid,
(meth)acrylic acid derivatives, such as (meth)acrylonitrile and
(meth)acrylic amide, vinyl ethers, such as vinyl methyl ether and
vinyl phenyl ether.
[0024] More preferably, said LDPE copolymer of ethylene with at
least polar comonomer(s) is a LDPE copolymer of ethylene with one
or more of vinyl esters of monocarboxylic acids having 1 to 4
carbon atoms, such as vinyl acetate, or of (meth)acrylates of
alcohols having 1 to 4 carbon atoms, or of a mixture thereof,
preferably of vinyl acetate, methyl (meth)acrylate, ethyl
(meth)acrylate or butyl (meth)acrylate. The preferred subgroup of
said LDPE copolymer of ethylene with at least polar comonomer(s) is
a LDPE copolymer of ethylene with at least vinyl acetate, LDPE
copolymer of ethylene with at least methyl acrylate, a LDPE
copolymer of ethylene with at least ethyl acrylate or a LDPE
copolymer of ethylene with at least butyl acrylate, or any mixture
thereof. The term "(meth)acrylic acid" and "(meth)acrylate" are
intended to embrace both acrylic acid and methacrylic acid and,
respectively "methacrylate" and "acrylate".
[0025] The content of polar comonomer in said LDPE copolymer of
ethylene with at least polar comonomer(s) as defined above, that is
most preferable as said polymer component (a) is not limited and
may be of up to 70 wt %, preferably of 0.5 to 35 wt %, more
preferably of 1.0 to 35 wt %, of the total amount of said LDPE
copolymer.
[0026] High pressure polymerisation for producing said LDPE homo or
copolymer and the subgroups as defined above, is a well known
technology in the polymer field and can be effected in a tubular or
an autoclave reactor, preferably, in a tubular reactor. The high
pressure polymerisation is carried out suitably in a known manner,
e.g. at temperature range from 80 to 350.degree. C. and pressure of
from 100 to 400 MPa typically in the presence of an initiator of
the free radical/polymerisation reaction. Further details about
high pressure radical polymerisation are given in WO 93/08222. The
polymerisation of the high pressure process is generally performed
at pressures of from 1200 to 3500 bar and temperatures of from 150
to 350.degree. C.
[0027] The melt flow rate MFR.sub.2, of said polymer component (a)
may typically be at least 0.01 g/10 min, suitably at least 0.5 g/10
min, preferably at least 1.0 g/10 min, more preferably at least 2.0
g/10 min, even more preferably at least 3.0 g/10 min, when measured
according to ISO 1133, 2.16 kg load, 190.degree. C. The upper limit
MFR.sub.2 of said polymer component (a) is not limited and may be
e.g. up 50 g/10 min, such as up to 30 g/10 min, preferably up to 20
g/10 min, more preferably up to 15 g/10 min, when determined as
defined above.
[0028] The melt flow rate MFR.sub.21 of said polymer composition
(including carbon black (b)) may preferably be at least 0.1 g/10
min, more preferably at least 0.5 g/10 min, especially preferably
at least 1.0 g/10 min, even more preferably at least 2.0 g/10 min
when measured according to ISO 1133, 21.6 kg load, at 125.degree.
C. The upper limit MFR.sub.21 of said polymer composition may
preferably be up to 20 g/10 min, more preferably up to 15 g/10 min,
even more preferably up to 10 g/10 min when measured according to
ISO 1133, 21.6 kg load, at 125.degree. C.
[0029] Any of the above lower limits may be combined with any of
the above upper limits to form another preferred range for
MFR.sub.2 or MFR.sub.21, respectively.
[0030] It is preferred that said semiconductive polymer of the
invention is in the form of polymer powder or, preferably, of
pellets. The term pellets include herein granules and pellets of
any shape and type and are very well known and can be produced in
known manner using the conventional pelletising equipment.
[0031] In a further preferred embodiment the semiconductive polymer
composition is crosslinkable via radical reaction or crosslinking
via silane groups. In case of said semiconductive polymer
composition is crosslinkable via silane groups said silane groups
can be introduced into the polymer structure by copolymerisation of
monomers, such as olefin monomers, with silane-moiety bearing
comonomers, or by grafting crosslinkable silane-moieties bearing
compounds, such as unsaturated silane compounds with hydrolysable
silane group(s), onto a polymer. Both methods are well known in the
art. Grafting is usually performed by radical reaction using free
radical generating agents. Both for copolymerisation and for
grafting methods an unsaturated silane compound which is
represented by the formula
RSiR'.sub.nY.sub.3-n
wherein R is an ethylenically unsaturated hydrocarbyl or
hydrocarbyloxy group, R' is an aliphatic, saturated hydrocarbyl
group, Y is a hydrolysable organic group, and n is 0, 1 or 2. If
there is more than one Y group, these groups do not have to be
identical. Special examples of the unsaturated silane compound are
those wherein R is vinyl, allyl, isopropenyl, butenyl, cyclohexenyl
or gamma-(meth)acryloxypropyl, Y is methoxy, ethoxy, formyloxy,
acetoxy, propionyloxy or an alkyl or arylamino group, and R1 is a
methyl, ethyl, propyl, decyl or phenyl group.
[0032] An especially preferred unsaturated silane compound is
represented by the formula CH2.dbd.CHSi(OA).sub.3, wherein A is a
hydrocarbyl group having 1-8 carbon atoms, preferably 1-4 carbon
atoms. The most preferred compounds are vinyltrimethoxy silane,
vinyl dimethoxyethoxy silane, vinyltriethoxy silane,
gamma-(meth)acryloxypropyl silane, and vinyltriacetoxy silane.
[0033] Preferred crosslinkable semiconductive polymer composition
of the invention is crosslinkable via radical reaction, whereby
said the semiconductive polymer composition comprises a
cross-linking agent, preferably peroxide which preferably
constitutes between 0-8 wt %, preferably of from 0.1 to 5 wt %, of
the semiconductive polymer composition. Preferred peroxides used
for cross-linking are di-tert-amylperoxide,
2,5-di(tert-butylperoxy)-2,5-dimethyl-3-hexyne,
2,5-di(tert-butylperoxy)-2,5-dimethylhexane,
tert-butylcumylperoxide, di(tert-butyl)peroxide, dicumylperoxide,
di(tert-butylperoxy-isopropyl)benzene, butyl-4,4-bis(tert-butyl
peroxy)valerate,
1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane,
tert-butylperoxybenzoate, dibenzoyl-peroxide. Further, the addition
of the cross-linking agent is preferably effected after an optional
subsequent process step of pellet formation, as described further
below.
[0034] Said semiconductive polymer composition may comprise further
components, typically additives, such as antioxidants, crosslinking
boosters, scorch retardants, processing aids, fillers, coupling
agents, ultraviolet absorbers, stabilizers, antistatic agents,
nucleating agents, slip agents, plasticizers, lubricants, viscosity
control agents, tackifiers, anti-blocking agents, surfactants,
extender oils, acid scavengers and/or metal deactivators. The
content of said additives may preferably range from 0 to 8 wt %,
based on the total weight of the semiconductive polymer
composition.
[0035] Examples of such antioxidants are as follows, but are not
limited to: hindered phenols such as
tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)]methane;
bis[(beta-(3,5-ditert-butyl-4-hydroxybenzyl)-methylcarboxyethyl)]sulphide-
, 4,4'-thiobis-(2-methyl-6-tert-butylphenol),
4,4'-thiobis(2-tert-butyl-5-methylphenol),
2,2'-thiobis(4-methyl-6-tert-butylphenol), and thiodiethylene
bis(3,5-di-tert-butyl-4-hydroxy)-hydrocinnamate; phosphites and
phosphonites such as tris(2,4-di-tert-butyl-phenyl)phosphite and
di-tert-butylphenyl-phosphonite; thio compounds such as
dilaurylthiodipropionate, dimyristylthiodipropionate, and
distearylthiodipropionate; various siloxanes; polymerized
2,2,4-trimethyl-1,2-dihydroquinoline,
n,n'-bis(1,4-dimethylpentyl-p-phenylenediamine), alkylated
diphenylamines, 4,4'-bis(alpha,alpha-dimethylbenzyl)diphenylamine,
diphenyl-p-phenylenediamine, mixed di-aryl-p-phenylenediamines, and
other hindered amine antidegradants or stabilizers. Antioxidants
can be used in amounts of about 0.1 to about 5 percent by weight
based on the weight of the composition.
[0036] Examples of further fillers as additives are as follows:
clays, precipitated silica and silicates, fumed silica, calcium
carbonate, ground minerals, and further carbon blacks. Fillers can
be used in amounts ranging from less than about 0.01 to more than
about 50 percent by weight based on the weight of the
composition.
[0037] A second aspect of the present invention relates to a
process for preparing a semiconductive polymer composition
comprising the steps of: i) introducing 30-90 wt % of a polymer
component (a) as defined above and 0-8 wt % additives in a mixer
device and mixing the polymer component (a) and additives at
elevated temperature such that a polymer melt is obtained; ii)
adding 10-70 wt % of a carbon black as defined above to the polymer
melt and further mixing of the polymer melt to obtain a
semiconductive polymer mixture, and iii) extruding and pelletising
the obtained polymer mixture to obtain said semiconductive polymer
composition.
[0038] Preferably, the carbon black is added to the polymer melt in
at least two subsequent addition steps. It is even more preferred
when in the first addition step at least 2/3 of the total amount of
carbon black is added to the melt, and in the second addition step
the remainder of the total amount of carbon black is added to the
melt. By carrying out the method according to the invention in this
way, a very advantageous particle size distribution and mixing of
the carbon black in the polymer mixture is obtained.
[0039] It is preferred when the semiconductive composition,
depending on the desired end application, comprises not more than
200, more preferably not more than 100, even more preferably not
more than 50, especially preferably not more than 25, and in very
demanding end applications even not more than 15, particles per
m.sup.2 having a width larger than 150 .mu.m at the half height of
said particle protruding from the surface of the tape sample.
[0040] Preferably, the semiconductive composition comprises not
more than 9, more preferably not more than 8, most preferably not
more than 6 particles per m.sup.2 having a width larger than 200
.mu.m at the half height of said particle protruding from the
surface of the tape sample.
[0041] In a most preferred embodiment the semiconductive
composition comprises not more than 10 particles per m.sup.2 having
a width larger than 150 .mu.m at the half height of said particle
protruding from the surface of the tape sample and not more than 3
particles per m.sup.2 having a width larger than 200 .mu.m at the
half height of said particle protruding from the surface of the
tape sample.
[0042] The carbon black is preferably furnace carbon black and/or
the polymer component is preferably a homo- or copolymer of
polyethylene. Any type of furnace carbon black may be used, but in
a preferred embodiment of the invention a modified carbon black
based on N550 type is employed.
[0043] In this regard it is further noted that in the method
according to the invention the carbon black, the polymer component
and the additives may be, and most preferably are, as have been
described above in relation to the polymer composition. The mixing
after introducing the polymer component (a) and optional additives
in the preparation process is effected at elevated temperature and
results typically in melt mixing, typically more than 10.degree. C.
above, preferably more than 25.degree. C., above the melting point
of the polymer component(s) and below the undesired degradation
temperature of the components, preferably below 250.degree. C.,
more preferably below 220.degree. C., more preferably of from 155
to 210.degree. C., depending on the used polymer material.
Preferably said preparation process of the invention further
comprises a step of pelletising the obtained polymer mixture.
Pelletising can be effected in well known manner using a
conventional pelletising equipment, such as preferably conventional
pelletising extruder which is integrated to said mixer device. The
process of the invention can be operated in batch wise or in
continuous manner.
[0044] Apparatuses used for carrying out the method of the
invention are for example single screw or twin screw mixer or a
kneading extruder, or a combination thereof, which is preferably
integrated in a pelletising device. The apparatus(es) may be
operated in batch wise or, preferably, in continuous manner. The
process may comprise a further subsequent sieving step before
preferable pelletising step which is also conventionally used in
the prior art in the preparation of semiconductive polymer
compositions to limit the number of large particles. Said sieving
step has normally no or minor effect on particle size distribution
as now provided by the present invention. A third aspect of the
present invention relates to a semiconductive polymer composition
or pellets thereof which can be obtained by the method as described
above.
[0045] A fourth aspect of the invention relates to the use of the
semiconductive polymer or its pellets for the production of a
semiconductive layer of an electric power cable. A further aspect
of the present invention relates to an electric power cable
comprising at least one semiconductive layer, which layer comprises
a semiconductive polymer composition as described above.
[0046] Preferably, the power cable may comprise a conductor, an
inner semiconductive layer (a), an insulation layer (b) and an
outer semiconductive layer (c), each coated on the conductor in
this order, wherein at least one of the inner and outer
semiconductive layer(s) (a;c) comprises a semiconductive polyolefin
composition according to the present invention as described
above.
[0047] In a further preferred embodiment of the inventive power
cable both the inner (a) and outer (c) semiconductive layers,
comprise, more preferably consist of the semiconductive polyolefin
composition according to the present invention.
[0048] In a further preferable embodiment, at least one of the
inner and outer semiconductive layers (a;c) is crosslinkable,
preferably both inner (a) and outer (c) semiconductive layers are
crosslinkable.
[0049] According to another embodiment of the inventive power cable
the outer semiconductive layer (c) may be strippable or
non-strippable, preferably non-strippable, i.e. bonded. These terms
are known and describe the peeling property of the layer, which may
be desired or not depending on the end application. In case of
strippable semiconductive layer, the polymer (a) of the invention
is more polar having a content of polar comonomers of at least 20.0
wt %, such as at least 25.0 wt %, preferably at least 26.0 wt %
more preferably from 27.0 to 35.0 wt %, based on said polymer (a),
and may contain further polar polymer components to contribute to
strippability. Preferably the outer semiconductive, if present, is
non-strippable and has a content of polar comonomers of less than
25.0 wt %, preferably less than 20.0 wt %, more preferable of from
10.0 to 18.0 wt %. In some embodiments the polar comonomer content
as low as of 6.0 to 15.0 wt % based on said polymer (a) may be
desired. In both strippable and non-strippable cases the layer is
preferably crosslinkable.
[0050] The insulation layer (b) is well known in power cable
applications and can comprise any polymeric material suitable
and/or conventionally used for such insulation layer. Also the
insulation layer (b) is preferably crosslinkable. Accordingly, the
invention also provides a process for producing a power cable,
wherein the process comprises blending the semiconductive
polyolefin composition of the invention as defined above including
any subgroups thereof, optionally with other polymer components and
optionally with additives, above the melting point of at least the
major polymer component(s) of the obtained mixture, and extruding
the obtained melt mixture on a conductor for forming at least one
semiconductive polymer layer on a conductor for a power cable. The
processing temperatures and devices are well known in the art.
Preferably, said polyolefin composition of the invention is used in
form of pellets which are added to the mixing step and melt mixed.
Preferably, the semiconductive polyolefin composition is
co-extruded on the conductor together with one or more further
cable layer(s) forming polymeric composition(s), thus providing a
multilayered power cable, preferably a multilayered power cable as
defined above. After providing the layered power cable structure,
preferably the multilayered power cable as defined above, the
obtained cable is then crosslinked in a subsequent crosslinking
step, i.e. said cable preparation process comprises a further step
of crosslinking the obtained power cable as defined above, by
contacting said at least one semiconductive layer which comprises
said semiconductive polymer composition as defined above and which
layer is crosslinkable, with a crosslinking agent, which is
preferably a silanol condensation catalyst, in the presence of
water in case of crosslinking via silane groups, or with a
crosslinking agent which is preferably a peroxide in case of
crosslinking via radical reaction.
[0051] Preferably, said at least one semiconductive cable layer is
crosslinked during the preparation process of said cable via
radical reaction using a peroxide as the crosslinking agent.
[0052] Such crosslinking step is preferably effected as an
integrated subsequent step of the cable preparation process in a
crosslinking zone. Preferred peroxide-crosslinking can be effected
at the temperature of at least above 160.degree. C., preferably
above 170.degree. C. The crosslinked cable is then recovered and
further processed if needed.
[0053] In the alternative crosslinking via silane groups, said
layers comprising said semiconductive polymer composition of the
invention are preferably crosslinked using a silanol-condensation
catalyst which is preferably selected from carboxylates of metals,
such as tin, zinc, iron, lead and cobalt; from organic bases; from
inorganic acids; and from organic acids; more preferably from
carboxylates of metals, such as tin, zinc, iron, lead and cobalt,
or from organic acids, preferably from an organic sulphonic acid
having a formula Ar(SO.sub.3H).sub.x (II) wherein Ar is an aryl
group which may be substituted or non-substituted, and x being at
least 1, or a precursor of the sulphonic acid of formula (II)
including an acid anhydride thereof or a sulphonic acid of formula
(II) that has been provided with a hydrolysable protective
group(s), e.g. an acetyl group that is removable by hydrolysis.
Such organic sulphonic acids are described e.g. in EP736065, or
alternatively, in EP 1 309 631 and EP 1 309 632.
[0054] The crosslinking via silane groups is effected in an
elevated temperature, typically of below 100.degree. C., preferably
of below 80.degree. C., more preferably of between 60 to 80.degree.
C. If said preferable silanol condensation catalyst as defined
above is used, the crosslinking step is carried out in the presence
of water or steam, e.g. water vapor, or both, preferably at least
water vapor, as well known in the art. Said silane-crosslinking can
be effected in a conventional manner using a conventional
equipment.
[0055] Preferred crosslinking of cable is crosslinking via radical
reaction using a peroxide as defined above.
[0056] Thus said crosslinked cables obtainable by the above
crosslinking method via silane groups or, preferably via radical
reaction, are also provided.
Determination Methods
[0057] If not already specified in the above general description
the properties as defined above, in below examples and claims where
analysed using the following methods.
Wt %=weight percent
a) Melt Flow Rate
[0058] MFR.sub.2 is the melt flow rate of a polymer component (a)
and was measured according to ISO 1133, 2.16 kg load, at
190.degree. C. for polyethylene. MFR.sub.21 of a polymer
composition (including carbon black) was measured according to ISO
1133, 21.6 kg load, at 125.degree. C. for polyethylene.
b) Comonomer Content
[0059] Comonomer content was based on the polymerisable comonomer
units and was determined by using .sup.13C-NMR. The .sup.13C-NMR
spectra were recorded on Bruker 400 MHz spectrometer at 130.degree.
C. from samples dissolved in 1,2,4-trichlorobenzene/benzene-d6
(90/10 w/w).
[0060] In an alternative and comparable method for comonomer
content (wt %): was determined in a known manner based on Fourier
transform infrared spectroscopy (FTIR) determination calibrated
with .sup.13C-NMR. The peak for the comonomer was compared to the
peak of polyethylene (e.g. the peak for butyl acrylate at 3450
cm.sup.-1 was compared to the peak of polyethylene at 2020
cm.sup.-1 and the peak for silane at 945 was compared to the peak
of polyethylene at 2665 cm.sup.-1. The calibration with
.sup.13C-NMR is effected in a conventional manner which is well
documented in the literature. The weight-% was converted to mol-%
by calculation.
c) Mass Pellet Strength
[0061] The test is performed determined according to ASTM D1937-13.
This test method covers the determination of the mass strength of
pelleted carbon black. It is designed to determine the force
required to pack a cylindrical column with pelleted carbon black. A
sample of carbon black is placed in a vertical cylinder and pressed
with a plunger for 10 s after which the bottom of the cylinder is
opened, whereupon all of the carbon black either falls out the
bottom or forms a ring or bridge in the cylinder. The process is
repeated with a new sample until the minimum force required for the
carbon black to form a ring or bridge is found. Force is increased
with increments of 50 N. Once a bridge of pressed carbon black is
formed, the end point has been reached or exceeded. An additional
test is performed at a lower pressure to confirm that the end point
has not been exceeded. The end point is the lowest number of
newtons required to produce a ring or bridge of pressed carbon
black in the cylinder. The resultant force is called mass strength
and is reported in Newtons. The apparatus has the following
specifications:
Mass strength tester: flat compression cylinder of 50.8 mm
diameter. Calibrating block: made from a cylindrical wooden shaft,
47 mm in diameter and 250 mm long. Platform scale: 50 kg capacity
with a sensitivity of 0.1 kg for air pressure gage calibration.
Overflow cup to fit around cylinder: normally part of the apparatus
delivery. Spatula: with a straight edge of at least 55 mm.
[0062] Carbon black samples are taken in accordance with practices
ASTM D1799 or ASTM D1900.
d) Individual Pellet Hardness
[0063] Samples of carbon black are taken in accordance with
practice ASTM D1799 or Practice ASTM D1900. The parameters of
average crush strength (CSAV) and average value for the 5 hardest
pellets (M5H) are determined according to ASTM D5230-13. A sample
of carbon black is passed through two sieves to isolate a fraction
of uniform size. An appropriate amount of pellets from this portion
is selected and placed into the tester. The individual pellets are
pressed against a platen with a load cell for measuring force. As
pressure is applied the pellet will either break with a rapid force
reduction or the pellet will simply compress. The individual pellet
hardness is the maximum force prior to a force reduction of at
least 3 cN or the maximum force required to compress the pellet to
90%, whichever comes first. For determination of CSAV, 20 pellets
were tested and the average value was calculated. For determination
of M5H the average value of the crush strength of the five hardest
of the pellets taken from the determination of CSAV was
calculated.
e) Total and External Surface Area of Carbon Black by Nitrogen
Adsorption
[0064] The test method is performed according to ASTM D6556-10 and
covers the determination of the total surface area by the Brunauer,
Emmett, and Teller (B.E.T. NSA) theory of multilayer gas adsorption
behavior using multipoint determinations and the external surface
area based on the statistical thickness surface area method. The
total and external surface areas are measured by evaluating the
amount of nitrogen adsorbed, at liquid nitrogen temperature, by a
carbon black at several partial pressures of nitrogen. A minimum of
five data points evenly spaced in the 0.1 to 0.5 relative pressure
(P/Po) range is obtained. A data point consists of the relative
pressure of equilibrium and the total amount of nitrogen gas
adsorbed by the sample at that relative pressure. The adsorption
data is used to calculate the NSA and STSA values as described in
ASTM D6556-10. Carbon black samples are taken in accordance with
practices ASTM D1799 and ASTM D1900.
f) Iodine Adsorption Number of Carbon Black
[0065] The test method is performed according to ASTM D1510-13,
method A. A weighed sample of carbon black is treated with a
portion of standard iodine solution and the mixture shaken and
centrifuged. The excess iodine is then titrated with standard
sodium thiosulfate solution, and the adsorbed iodine is expressed
as a fraction of the total mass of black (g/kg). Samples of carbon
black are taken in accordance with practice ASTM D1799 or Practice
ASTM D1900.
g) Oil Absorption Number (OAN) of Carbon Black
[0066] The test method is performed according to ASTM D2414-13,
procedure A. Carbon black samples are taken in accordance with
practices ASTM D1799 and ASTM D1900. n-Dibutyl phthalate (DBP) oil
is added by means of a constant-rate buret (delivering 4+/-0.024
cm.sup.3/min.) to a sample of carbon black in the mixer chamber of
an absorptometer. The absorptometer is preferably equipped with a
torque measuring system that includes a micro-computer and software
to continuously record torque and oil volume with time. A testing
temperature of 23+/-5.degree. C. is maintained. As the sample
absorbs the oil, the mixture changes from a free-flowing state to a
semiplastic agglomeration, with an accompanying increase in
viscosity. This increased viscosity is transmitted to the torque
sensing system of the absorptometer. When the viscosity of the
mixture reaches a predetermined torque level, the absorptometer and
buret will shut off simultaneously. The volume of oil added is read
from the direct-reading buret. The volume of oil per unit mass of
carbon black is the oil absorption number (cm.sup.3/100 g).
h) Pellet Size Distribution of Carbon Black
[0067] A sample of carbon black is shaken in a sieve shaker to
separate the pellets by size with specified series of sieve screens
arranged with progressively smaller openings. The percentage, by
mass, of carbon black retained on each sieve is weighed to
calculate the pellet size distribution. U.S. standard sieves
(conforming to Specification E11), sieve nos. 10, 18, 35, 60, and
120 having openings respectively of 2000, 1000, 500, 250, and 125
.mu.m, shall be used. The sieves shall be 25 mm in height and 200
mm in diameter. Carbon black samples are taken in accordance with
practices ASTM D1799 and ASTM D1900.
j) Surface Smoothness Analysis (SSA) Method
[0068] The general definitions for the surface smoothness
properties of the semiconductive polymer composition of the
invention as given above and below in the claims, as well as given
in the examples below were determined using the sample and
determination method as described below.
[0069] For illustrative purposes a schematic overview of the test
apparatus is provided in FIG. 1. Herein, a tape 1 consisting of the
semiconductive polymer composition passes over a rod 2 at a given
speed and a light beam 3 coming from the light source 4 passes over
the tape 1 and this light beam 3 is captured by the camera 5. When
there is a particle 7 protruding from the surface of the tape 1,
the light beam 3 will be altered, which alteration will be recorded
by the camera 5. From this recording by the camera 5 it is possible
to calculate the height and the width of the particle protruding
from the surface of the tape. In this manner the amount, height and
width of the particles present in the tape can be measured.
[0070] This method is used to determine the surface smoothness,
i.e. the particles protruding outwards from the surface and thus
causing the roughness of the tape surface. It indicates the
smoothness of a polymer layer on a cable produced by (co)extrusion.
The method detects and measures the width of a protruding particle
at the half height of said protrusion thereof from the surface of
the tape. The test system is further generally described e.g. in WO
00/62014.
(i) Tape Sample Preparation
[0071] About 4 kg of pellets of a semiconductive polymeric
composition were taken and extruded into a form of tape sample
using Collin single screw of 20 mm and 25 D extruder (supplier
Collin) and following temperature settings at different sections,
starting from the inlet of the extruder: 95.degree. C., 120.degree.
C., 120.degree. C. and 125.degree. C. to obtain a temperature of
125.degree. C. of the polymer melt. The pressure before the
extrusion plate is typically 260 bar, residence time is kept
between 1 and 3 minutes and typical screw speed is 50 rpm,
depending on the polymer material as known for a skilled person.
Extruder die opening: 50 mm.times.1 mm, Thickness of the tape: 0.5
mm+/-10 .mu.m, Width of the tape: 20 mm+/-2 mm.
[0072] The tape is cooled with air to solidify it completely before
subjecting to a camera-scanning (detection) zone of the
SSA-instrument which locates at a distance of 50 cm from the outlet
of die. The measurement area: Camera of SSA-instrument scans the
tape surface while the tape moves with a given speed. The scanning
width is set to exclude the edge area of the tape. The scanning is
effected on along the tape to correspond to a measurement area of 1
m.sup.2. Further details are given below.
(ii) SSA Determination of the Tape Sample
[0073] The test is based on an optical inspection of the obtained
extruded tape that is passed in front of an optical scanner able to
scan even a large surface at high speed and with good resolution.
The SSA-instrument is fully computerised and during the operation
it automatically stores information about positions and sizes of
pips found for statistical evaluation. "Pip" means herein a smaller
burl with a height at least one order of magnitude higher than the
surrounding background roughness. It is standing alone and the
number per surface area is limited.
[0074] Height is the distance between the base line (=surface of
the tape) and the highest point of a pip. Half height is defined as
the width of the pip at 50% of its height (W50) measured from the
baseline. For the half height measurement the surface of the tape
sample is taken as the baseline. Pip is referred herein above and
below as a "particle protruding from the surface of the tape". And
thus the "half height of said particle protruding from the surface
of the tape sample" as used herein in the description and claims is
said half height width (W50). The instrument was SSA-analysing
instrument of Semyre Photonic Systems AB, Sweden. Service company
is Padax AB, Sweden. New supplier is OCS GmbH in Germany.
Hardware: PC via Image Pre Processor
Software: NOPINIT
[0075] Camera type: spectrophotograph camera from Dalsa with 2048
pixels, on-line camera with line frequency of 5000. Light source:
intensity regulated red LED, The width resolution of the pip
(particle): 10 .mu.m, The height resolution of the pip (particle):
1.5 .mu.m.
[0076] Tape speed in SSA-instrument: 50 mm/s The horizon of tape
surface is created of a rotating metal shaft. The light source and
camera are directly aligned with no angel with a focal point on the
horizon.
[0077] The scanning results are for 1 m.sup.2 of tape and expressed
as [0078] number of particles per m.sup.2 having a width larger
than 150 .mu.m at a half height of said particle protruding from
the tape surface (=baseline), [0079] number of particles per
m.sup.2 having a width larger than 200 .mu.m at a half height of
said particle protruding from the tape surface (=baseline).
[0080] The given values represent an average number of particles
obtained from 10 tape samples prepared and analysed for a
semiconductive composition under determination.
[0081] It is believed that when using the above principles the
SSA-method can be performed using another camera and set up-system
provided the particle sizes given in description and claims can be
detected and height at half width determined with corresponding
accuracy, would result in the same results as the above reference
SSA-method.
k) Volume Resistivity (VR)
[0082] The VR was measured according to ISO 3915 using the
four-point method and tape samples consisting of the test polymer
composition. The test tape was prepared herein as defined above
under "Surface smoothness" test. The resistivity was measured using
a conventional two electrode set-up, wherein the tape is arranged
between the electrodes.
[0083] The volume resistivity was calculated as follows: Volume
Resistivity (VR, ohmcm)=R.times.B.times.D wherein,
R=resistivity, ohm, B=breadth of tape, cm, D=thickness of tape, cm,
L=distance between the two electrodes, cm.
[0084] In the method used herein the distance was L=2.54 cm
[0085] The present invention will be further illustrated by means
of the following examples:
EXAMPLE 1
Semiconductive Polymer Composition of the Invention
[0086] 61.4 wt % of conventional ethylene butyl acrylate (EBA)
copolymer produced via radical polymerisation in a high pressure
tubular reactor, and the copolymer having the following properties:
MFR.sub.2 of 18 g/10 min (ISO 1133, load 21.6, 190.degree. C.),
butyl acrylate (BA) comonomer content of 14 wt %, melt temperature
of 110.degree. C., density 924 kg/m.sup.3 (ASTM D792), was fed
together with 0.4 wt % of commercially available antioxidant
(4,4'-bis(1,1'-dimethylbenzyl)diphenylamine) to the first hopper of
a Buss mixer, MDK/E 200, (commercially available from Buss with a
reciprocating co-kneader). The polymer component was mixed under
heating to a molten stage. The temperature profile in said mixer
for this test was as follows measured from the molten polymer
mixture: first section 104.degree. C.; second section 117.degree.
C., third section 159.degree. C., fourth section 201.degree. C. and
fifth section 208.degree. C. The carbon black (b) used for
preparing the semiconductive composition of this example had the
properties as shown in Table 1. The carbon black (CB1) (modified
N550 grade, (Birla carbon)) was added in two stages. The first part
of 27.5 wt % of the carbon black was fed to mixer before said
second section of 117.degree. C. and the rest, second part, of said
carbon black 10.7 wt % before said third section of 159.degree. C.
The total content of added carbon black was 38.2 wt %. The total
throughput of the mixer was 1200 kg/hrs and the screw speed of the
mixer was set at 121 rpm. The molten polymer mixture obtained from
the mixer was then transferred to a commercial extruder, available
from Berstorff, which was operating as an integrated unit with said
mixer to provide 150 bar pressure for filtering the molten polymer
through a 150 .mu.m mesh filter in a known manner. The operating
temperature of said extruder was approximately 220.degree. C. After
the filtration the polymer was pressed thorough an extrusion plate
for forming pellets thereof in conventional manner. After
pelletisation the pellets are dried and some of about 4 kg pellets
are taken out for tape sample preparation for the SSA-analysis as
defined above under Determination Methods in order to determine the
surface smoothness of the obtained material.
COMPARATIVE EXAMPLE 1
Comparative Semiconductive Polymer Composition
[0087] The preparation for producing this composition was effected
in the same way as described in Example 1 except that the type of
carbon black (CB2) was modified as specified in Table 1.
EXAMPLE 2
Semiconductive Polymer Composition of the Invention
[0088] The preparation for producing this composition was effected
in the same way as described in Example 1 except that the type of
carbon black (CB3) was modified as specified in Table 1.
EXAMPLE 3
Semiconductive Polymer Composition of the Invention
[0089] The preparation for producing this composition was effected
in the same way as described in Example 1 except that the type of
carbon black (CB4) was modified as specified in Table 1. The carbon
black was N550 grade, supplier: Orion carbon, former: Evonik.
COMPARATIVE EXAMPLE 2
Comparative Semiconductive Polymer Composition
[0090] The preparation for producing this composition was effected
in the same way as described in Example 3 except that the type of
carbon black (CB5) was modified as specified in Table 1.
TABLE-US-00001 TABLE 1 Ex- Ex- Ex- Compar- Compar- ample ample
ample ative ative 1 2 3 Example 1 Example 2 Unit CB1 CB3 CB4 CB2
CB5 CB properties Type -- N550 N550 N550 N550 N550 STSA m.sup.2/g
41 40 38 42 39 Iodine g/kg 45 45 41 45 41 adsorption number DBP
cm.sup.3/ 122 122 123 122 122 absorption 100 g number CB pellet
Tough- MPS N 156.91 137.29 78.45 284.39 313.81 properties ness
Individual CSAV cN 12.75 11.77 22.56 16.67 22.56 of CB pellet
(averaqe) pellets hardness M5H cN 20.59 20.59 29.42 N/A 46.09
(average for the 5 hardest pellets) CB pellet size mm 0.76 0.55 1.2
0.81 0.78 Compound Surface Pips > 150 .mu.m pcs/m.sup.2 10 40 9
226 500 properties roughness Pips > 200 .mu.m pcs/m.sup.2 2 6 1
10 70 VR at room temp Ohm cm 5 5 4 5 N/A CB content % 38.2 38.2
38.2 38.2 38.2
[0091] The samples were found to give different dispersion level of
carbon black in the compounds. CB1 (inventive sample) achieved the
highest dispersion level of carbon black which results in the
lowest surface protuberance. This is due to the improved pellet
parameters as also reported in Table 1. CB2 (comparative sample)
gave no preferable (or too high) level of surface protuberance for
semiconductive applications due to too high value of mass pellet
strength (MPS). CB3 (inventive sample) gave much better carbon
black dispersion level than CB2. The surface smoothness level is
acceptable but not as high as that of the composition of Example
1.
[0092] CB4 gives excellent dispersion level of carbon black which
results in very good surface smoothness, and yet the volume
resistivity value is the lowest. This is due to the appropriate
pellet parameters and also the combination of low mass pellet
strength (MPS) and large pellet size. On the other hand, CB5 gives
no preferable (or the highest) level of surface protuberance for
semiconductive applications. This is due to too high values of MPS
and M5H.
[0093] At the same time the semiconductive polymer compositions of
the present invention provide excellent surface smoothness
according to the above SSA test. Example 1 and Example 3 provided
compositions with excellent surface smoothness as the respective
carbon blacks CB1 and CB4 met the specifications of the invention
with respect to pellet crush resistance. CB 2 had an excessive mass
pellet strength so that dispersion in the polymer resin were poor.
As a result, a large number of particles with a size of >150
.mu.m and >200 .mu.m were detected in a tape sample according to
the SSA test. The composition of Example 2 has a fair compromise
between surface smoothness and volume resistivity. Due to a rather
small average pellet size, the surface smoothness is not as high as
with CB1 in Example 1.
[0094] A comparison between Example 3 and Comparative Example 2
shows that CB5 has excessively high pellet crush resistance (MPS
and M5H) and thus the surface smoothness was poor due to a large
number of particles with a size of >150 .mu.m and >200 .mu.m
detected in a tape sample according to the SSA test.
[0095] The above examples show that a semiconductive polymer
composition containing a specific, modified carbon black meeting
the specifications of the invention regarding pellet crush
resistance lead to a composition having a superior overall
performance regarding surface smoothness, dispersibility of the
carbon black in the polymer composition and electric resistance
(volume resistivity) which is important in semiconductive
applications, especially if the compositions are used for the
production of a semiconductive layer of an electric power
cable.
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