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DESCRIPTION
~ PROCESS FOR PRODUCING FLAME-RETARDANT,
SILANE-CROSSLINKED POLYOLEFIN
TECHNICAL FIELD
This invention relates to a silane-
crosslinking process for producing a flame-retardant,
silane-crosslinked polyolefin in one step using a carrier
polymer cont~; n; ng an organic unsaturated silane or the
like at a high concentration in the silane-crosslinking
of a flame-retardant polyolefin cont~; ni ng a silanol-
co~nsing catalyst and a flame-retardant.
BACKGROUND ART
In order to impart a flame-retardancy to a
polyolefin composition which has been often used in coat-
ing wires, cables and the like; hoses; sheets; injectionmoldings; and the like, a halogen compound and diantimony
trioxide have heretofore been added to the polyolefin to
achieve the flame-retardancy. However, these composi-
tions are halogen-containing compositions, so that halo-
gen gases, which are not only harmful in themselves butalso the cause of metal corrosion, are generated when
they are burned. Therefore, these compositions have not
been desired. In addition, the amount of smoke yenerated
is large, the visibility b~r~ -~ bad and the people have
been markedly prevented from taking refuge from a fire
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and from fighting a fire.
In particular, from a safety aspect, it has
recentl~ been strongly required that such a halogen-con-
taining gas be not generated. Under such circumstances,
attention has been paid to an inorganic flame-retardant
using a hydrated metal compound which generates very
little fumes and is very little harmful.
In recent years, a resin composite cont~ n; ng
magnesium hydroxide, aluminum hydroxide or the like has
been put to practical use for imparting flame-retardancy;
however, these flame-retardant resin compositions can
prevent the generation of halogen-cont~i n; ng gases but
have a problem in that there is caused a so-called drip
ph~nom~on that the resin compositions melt and drip dur-
ing the firing or in that the resin compositions areinferior in shape-ret~;n~hility at high temperatures.
In order to improve the flame-retardancy and
also the heat resistance to solve the above problem,
there is a method of crosslinking the resin, and this is
disclosed in, for example, JP-B-57-24373, JP-B-57-26620
and the like. ~owever, the method described therein
requires a large scale crosslinking apparatus for chemi-
cal crosslinking, crosslinking with electron beams or the
like and hence the cost of the equipment per se and the
expenses for subsequent operation, mainten~nce, control
and the like of the apparatus are increased, resulting in
an increase of the cost of the composition. Furthermore,
as stated in JP-A-60-101129 and JP-A-60-147463, in the
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case of a flame-retardant, crosslinked composition char-
acterized in that a flame-retardant is added to a silane-
grafted polyolefin resin and this silane-grafted poly-
ole~in resin is finally crosslinked, only the silane-
grafted polyolefin resin whose crosslinking has beencontrolled has been able to be used for preventing prema-
ture crosslinking in bl~A; ng and kneading the flame-
retardant with the silane-grafted polyolefin resin. As a
result, it has been impossible to increase the degree of
crosslinking of the silane-grafted polyole~in resin and
hence the heat resistance has been insufficient.
The above method requires at least two reac-
tion steps, namely, the silane-grafting reaction step and
the silanol-condensing reaction step. Accordingly, at
least two extrusion steps must be carried out and an
economical difficulty of the final product is unavoid-
able.
Furthermore, the Monosil TM process method is
known as a one-step process. However, this method re-
quires a liquid-~AA;~g means for injecting an organic
unsaturated silane in the form of a liquid into an
extruder and has problems of slippage and metering fail-
ure. The extruder is also required to have a large L/D
and be of an expensive and special type for uniformly
dispersing a small amount of an additive, and hence, an
economical problem is unavoidable. In addition, a very
sophisticated technique is necessary for the extrusion
and hence there have been no comm~cialized method for
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flame-retardant type.
In addition, JP-A-3-167229 discloses, as a
one-step process, a silane-crosslinking method in which a
silane has been introduced into a solid carrier polymer.
However, this method uses a porous polymer or EVA as the
solid carrier polymer, and in this method, in addition to
the silane and the free-radical generator, an additive
such as a silanol-condensing catalyst, an antioxidant or
the like is also introduced into the solid carrier poly-
1~ mer. Therefore, an oligomer is formed by condensation ofthe silane or the crosslinking is inhibited by trapping
the radical, whereby such problems are caused that cross-
linking efficiency and storability are inferior. In said
Japanese publication, no flame-retardant type is men-
tioned.
This invention has solved these problems andaims at providing a silane-crosslinking method comprising
silane-crosslinking a flame-retardant polyolefin contain-
ing a silanol-condensing catalyst and a flame-retardant,
wherein a flame-retardant, silane-crosslinked polyolefin
is produced in one step using a carrier polymer contain-
ing an organic unsaturated silane or the like at a high
concentration.
~ISCLOSURE OF T~E INVENTION
This invention is a process for producing a
f~ame-retardant, silane-crosslinked polyolefin which
comprises melt-mixi ng (l) a flame-retardant polyolefin
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comprising 100 parts by weight of an a-olefin homopolymer
or copolymer having a density of not more than 0.92
~ g/cm3, 50 to 200 parts by weight of a flame-retardant,
0.5 to 10 parts by weight of a water-absorbing agent and
a catalytic amount of a silanol-condensing catalyst with
(2) a su~stantially water-free carrier polymer comprising
a free-radical generator and an organic unsaturated
silane represented by the general formula RR'SiYz in
which R is a monovalent olefinically unsaturated hydro-
carbon group; Y is a hydrolyzable organic group; and R'is a monovalent hydrocarbon group other than the
olefinically unsaturated hydrocarbon group or the same as
Y, at a higher temperature than the crystalline melting
point of the base polymer of the flame-retardant
polyolefin to subject them to reaction; subse~uently,
contacting the reaction mixture with water to subject the
reaction mixture to crosslinking. Preferably, this
invention provides the above process for producing a
flame-retardant, silane-crosslinked polyolefin, charac-
terized in that the a-olefin homopolymer or copolymer is
a homopolymer of an a-olefin such as ethylene, propylene
or the like, or a crystalline, block or random copolymer
of an a-olefin with at least one other a-olefins, or a
copolymer of a major amount of an a-ole~in with a minor
amount o~ at least one polar monomer selected from vinyl
acetate, maleic anhydride, acrylic acid and the like or a
mixture of them and has a density of not more than 0.92
g/cm3; the carrier polymer is a polymer selected from the
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group consisting of an ethylene-ethyl acrylate copolymer
(EEA), an ethylene-methyl methacrylate copolymer (EMMA),
a hydrogenated block copolymer obt~ine.~ by hydrogenating
a block copolymer consisting of at least one polymer
block comprising mainly a vinyl aromatic compound and at
least one polymer block comprising mainly a conjugated
diene compound and a mixture of them; the amount of the
carrier polymer added is 1 to 5% by weight; the flame-
retardant is a magnesium hydroxide or all~m;nllm hydroxide
surface~treated with at least one silane-coupling agent,
a silicone derivative, a fatty acid or a metal salt of a
fatty acid; and the water-absorbing agent is quicklime
having an average particle size of 1 to 5 ,um surface-
treated with at least one silane-coupling agent, a sili-
cone derivative, a fatty acid or a metal salt of a fattyacid.
This invention is explained below in detail.
The a-olefin homopolymer or copolymer used in
this invention is a homopolymer of an a-olefin such as
ethylene, propylene or the like; a crystalline, block or
random copolymer of an a-olefin with at least one other
a-olefin, for example, a crystalline propylene-ethylene
block copolymer, an ethylene-butene-l random copolymer or
a propylene-butene-1 random copolymer; a copolymer of a
major amount of an a-olefin with a minor amount of at
least one polar m~nom~ such as vinyl acetate, maleic
anhydride, acrylic acid or the like (including graft
copolymers); or a mixture thereof, and has a density of
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not more than 0.92 g/cm3. When the density exceeds 0.92
g/cm3, the compatlbility of the flame-retardant with the
a-olefin homopolymer or copolymer becomes inferior and
the flame-retardancy b~comes low.
The flame-retardant used in this invention is
a compound-having a decomposition-starting temperature in
the range o~ from 150~C to 450~C and represented by the
general formula MmOn-XHzO in which M is a metal, m and n
are integers of l or more determined by the valency of
the metal M and X is a numeral showing the crystal water
content, or a double salt cont~ n ~ ng said compound Spe-
cific examples of said compound include alllm;n~lm hydrox-
ide (Al203-3HzO or Al(OH)3), magnesium hydroxide (MgO-HzO
or Mg(OH)z), calcium hydroxide (CaO-HzO or Ca(OH)2), bari-
um hydroxide (BaO-HzO or BaO-9HzO), zirconium oxide hy-
drate ~ZrO-nH20), tin oxide hydrate (SnO-H20), basic
magnesium carbonate (3MgCO3-Mg(OH)z-3HzO), hydrotalcite
(6MgO-A1203.HzO), dawsonite (NazCO3-Al203-nHzO) and borax
(Na20-B20s.5H20), and also include zinc borate, zinc meta-
borate, barium metaborate, zinc carbonate, calcium magne-
sium carbonate, calcium carbonate, barium carbonate,
molybdenum oxide, red phosphorus, triethyl phosphate,
tricresyl phosphate, triphenyl phosphate, cresylphenyl
phosphate, octyldiphenyl phosphate, ethyl diethylenephos-
phate, butyl dihydroxypropylenephosphate, disodium ethyl-
~ ~n~phnsphate, ~mo~; um polyphosphate, melamine phosphate,
guanidine phosphate and the like. These may be used
alone or in combination of two or more, and magnesium
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hydroxide and aluminum hydroxide are preferred.
The flame-retardant is preferably one surface-
treated with (i) a silane coupling agent, (ii) a silicone
derivative, (iii) a fatty acid, (iv) a metal salt of a
fatty acid, or the like.
The silane-coupling agent.(i) is one having,
at one end of the molecule, a reactive group (methoxy
group, ethoxy group, carboxyl group, cellosolve group or
the like) which reacts with an inorganic material, and
generally, this has a trifunctional group in many cases;
however, those having a difunctional or monofunctional
group as the above reactive group may also be used. At
the other end, said silane-coupling agent has a reactive
group (vinyl group, epoxy group, methacryl group, amino
~ 15 group, mercapto group or the like) which chemically bonds
to the resin side which is an organic material, and the
skeleton of the main chain thereof is an alkoxyoligomer.
The silicone derivative (ii) is of such a type
that a part of the methyl group of dimethylpolysiloxane
2~ has been replaced by various organic groups, and the
various modifying organic groups include a wide variety
of modifying group derivatives, for example, modifying
groups for the purpose of i~ ~vlng compatibility, hydro-
philic property, lubricity, water-repellency and the like
such as a-methylstyrene group, ~-olefin group, polyether
group, alcohol group, fluoroalkyl group and the like;
modifying groups for the purpose of il..~lovlng the reac-
tivity and moisture-absorptivity such as amino group,
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mercapto group, epoxy group, carboxyl group and the like;
modifying groups for the purpose of releasing and luster-
- ing which has been substituted by a higher fatty acid,
carnauba wax and an amide; and the like.
The fatty acid (iii) includes saturated and
unsaturated fatty acids having 6 to 22 carbon atoms, and
specifically, stearic acid and oleic acid which have 18
carbon atoms are mentioned.
The metal salt (iv) of a fatty acid includes
metal soap in which the above-mentioned fatty acids are
bonded to metals, and specifically, sodium stearate,
potassium stearate, sodium oleate, potassium oleate and
the like are mentioned. Of course, the fatty acid may be
of linear chain saturated type or unsaturated type, and
those in which the side chain portion is bonded to the
metal are, of course, effective.
When the flame-retardant surface-treated with
one of (i) to (iv) or a mixture thereof is blended in an
amount of 50 parts by weight to 200 parts by weight, it
is compatible with or reacts with a low crystallinity
polymer having a low density, and even when a large
amount thereof is blended, it is possible to easily form
a composition excellent in low-temperature characteris-
tics, flexibility and processability. However, when the
amount is less than 50 parts by weight, the flame-retard-
ing effect thereof is remarkably lowered. When the
amount exceeds 200 parts by weight, the fIame-retardancy
is remarkably increased, but the mechanical characteris-
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tics including flexibility, low-temperature characteris-
tics and the like are deteriorated and the processability
becomes extremely bad.
The water-absorbing agent used in this inven-
tion is ~uicklime (calcium oxide), magnesium oxide, bari-
_ um oxide, all~m;n~lm oxide, magnesium-al~m;nl-m-oxide,
m~n~ ~ium~all lm; ~llm ~ hydroxide-carbonate, silica gel,
calcium sulfate anhydride or copper sulfate anhydride.
~uicklime is preferred, and those surface-treated with
the above (i) to (iv) components or a mixture thereof are
more preferable. This water-absorbing agent is added to
reduce the amount of water att~che~ to the flame-retar-
dant polyolefin. This is because when water is attached
to the flame-retardant polyolefin, premature crosslinking
is caused in the silane-crosslinking step. The amount of
water causing this premature crosslinking is about 100
ppm or more, and in order to control the amount of water
attached to less than 100 ppm, the water-absorbing agent
must be added in an amount of 0.5 to 10 parts by weight.
When the amount is less than 0 5 part by weight, the
amount of water attached b~co~ 100 ppm or more and
premature crosslinking is caused. Even if quicklime is
used in an amount of more than 10 parts by weight, the
effect of reducing the amount of water attached is not
increased and rather deterioration of mechanical charac-
teristics is caused in some cases. In order to make the
dispersibility in the flame-retardant polyolefin better
and obtain a more uniform effect of absorbing water
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attached, a water-absorbing agent having an average
particle size of 1 to 5 ,um is preferable.
~ The silanol-condensing catalyst used in this
invention includes organometallic compounds such as dibu-
tyltin dilaurate, stannous acetate, dibutyltin diacetate,
dibutyltin dioctoate, lead naphthenate, zinc caprylate,
cobalt naphthenate, tetrabutyl titanate, lead stearate,
zinc stearate, cadmium stearate, barium stearate, calcium
stearate and the like. The amount of the organometallic
compound added is 0.01 to 0.1% by weight, preferably 0.03
to 0.07% by weight, based on the total weight of the
flame-retardant, silane-crosslinked polyolefin. When the
amount is less than 0.01% by weight, the crosslinking
reaction does not proceed sufficiently, and when the
amount is more than 0.1% by weight, crosslinking proceeds
locally in an extruder when the composition is extruded
and the appearance is extremely deteriorated. The sila-
nol-condensing catalyst must be incorporated into the
flame-retardant polyolefin. This is because when the
silanol-con~n~ing catalyst is incorporated into the
carrier polymer impregnated with the organic unsaturated
silane or free-radical generator, the formation of an
oligomer due to the condensation of the silane is accel-
erated and the appearance is deteriorated.
The organic unsaturated silane used in this
invention is grafted on a base resin for forming mutual
crosslinking sites of the base resins. The organic
unsaturated silane used in this invention is a compound
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12
represented by the general formula RR'SiY2 in which R is
a monovalent olefinically unsaturated hydrocarbon group,
Y is a hydrolyzable organic group and R' is a monovalent
hydrocarbon group other than olefinically unsaturated
hydrocarbon groups or is the same as Y.
It is preferable to use an organic unsaturated
silane represented by the general formula RSiY3 which is
the case where R' is the same as Y, and there can be
used, for example, vinyltrimethoxysilane, vinyltriethoxy-
silane, vinyltributoxysilane, allyltrimethoxysilane,allyltriethoxysilane and the like. The amount of the
organic unsaturated silane added is 0.1 to 2g~ by weight,
preferably 0.4 to 1% by weight, based on the total weight
of the flame-retardant, silane-crosslinked polyolefin.
When the amount is less than 0.1% by weight, a sufficient
grafting is not caused and when the amount is more than
2~ by weight, molding failure i8 caused and the process
becomes uneconomical.
The free-radical generator used in this inven-
2~ tion serves as an initiator for silane-grafting reaction.
The free-r~dical generator used in this invention in-
cludes various organic peroxides and peresters having a
strong polymerization-initiating action, and there are
specifically mentioned, for example, dicumyl peroxide,
a, a ~ -bis~t-butylperoxydiisopropyl)benzene, di-t-butyl
peroxide, t-butylcumyl peroxide, dibenzoyl peroxide, 2,5-
dimethyl-2,5-bis(t-butylperoxy)h~x~ne, t-butyl peroxypi-
valate, t-butyl peroxy-2-ethylh~x~noate and the like.
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The amount of the free-radical generator added is 0.01 to
0.1~ by weight, preferably 0.02 to 0.05% by weight, based
- on the total weight of the flame-retardant, silane-cross-
linked polyolefin. When the amount is less than 0.01~ by
weight, the silane-grafting reaction does not proceed
sufficiently and when the amount is more than 0.1~ by
weight, the extrusion-processability is deteriorated and
the mold surface becomes bad.
By swelling the carrier polymer used in this
invention with a li~uid mixture in which the free-radical
generator is dissolved in the silane, the free-radical
generator and the silane can be incorporated into the
carrier polymer. At this time, for the incorporation at
a high concentration, it is necessary to preheat the
carrier polymer. However, the preheating must be ef-
fected at a temperature not higher than the crystalline
melting point so that the polymer is not melted. This is
because when the preheating is effected at a temperature
higher than the crystalline melting point, pellets are
melted and the workability is impeded thereby.
In addition, the carrier polymer must be in
the form of particles and must be a solid compatible with
the flame-retardant polyolefin and the silane, The
compatibility referred to herein means that the carrier
polymer must not easily react with the silane and must be
dispersible or soluble in the flame-retardant polyolefin.
Suitable carrier polymers are non-moisture-absorbable.
That is, it is preferable that the absorption of water is
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14
relatively slow in order to m;n~m; ze the possibility of
premature hydrolysis and condensation of the silane. In
any case, substantially no water should be present in the
carrier polymer. The carrier polymer used in this in-
vention is usually in the form of granules or pellets,and pellets are preferred.
The carrier polymer used in this invention
includes, for example, ethylene-ethyl acrylate copolymer
(EEA~, ethylene-methyl methacrylate copolymer (EMMA),
ethylene-vinyl acetate copolymer (EVA~, ethylene-propyl-
ene copolymer (EPR), ethylene-propylene-diene copolymer
(EPDM), a hydrogenated block copolymer obtained by hydro-
genati~g a block copolymer consisting of at least one
polymer block comprising mainly a vinyl aromatic compound
and at least one polymer block comprising mainly a conju-
gated diene compound such as hydrogenated styrene-iso-
prene block copolymer (SEPS), hydrogenated styrene-buta-
diene block copolymer (SEBS) or the like, and mixtures
thereof.
The amount of the carrier polymer added is 1
to 5% by weight based on the total weight of the flame-
retardant, silane-crosslinked polyolefin. When the
amount is less than l~ by weight, no sufficient grafting
is caused, and when the amount is more than 5~ by weight,
molding failure is caused and the process is not ~on~m'-
cal.
The flame-retardant polyolefin may, if neces-
sary, have added thereto as other additives, conventional
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additives such as antioxidant, neutralizing agent, ultra-
violet absorber, antistatic agent, pigment, dispersant,
viscosity improver, metal deterioration inhibitor, fungi-
cide, fluidity-controlling agent and the like, and may
further contain other synthetic resins.
Incidentally, in this invention, the silanol-
~on~nsing catalyst is added to the base polymer; how-
ever, in order to make the degree of dispersion of the
silanol-condensing catalyst better, there may be used a
carrier polymer other than the carrier polymer impregnat-
ed with the above-mentioned organic unsaturated silane
and free-radical initiator, for example, a previously
melt-m;x~ mixture of a polyolefin resin with a silanol-
condensing catalyst. In this case, it follows that the
base polymer and two kinds of carrier polymers are intro-
duced into a molding machine such as an extruder or the
like in a suitable bl~n~ing proportion.
BEST MODE FOR CARRYING OUT THE INVENTION
Examples are shown below to explain this
2~ invention in more detail.
<<Production of flame-retardant polyolefin>>
According to the compounding proportions shown
in Table 1, all the components shown in Table 1 were
mixed and kn~A~ at a temperature of 160-180~C using a
Banbury mixer to be pelletized.
<<Production of carrier polymer>>
According to the compounding proportions shown
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16
in Table 2, first of all, the carrier polymer was intro-
duced into a super mixer and stirred and mixed and then
preheated to 80~C. Subsequently, a liquid mixture ob-
tained by dissolving a free-radical initiator in an un-
saturated silane was introduced in~o a super mixer andthe carrier polymer was impregnated with the liquid mix-
ture with stirring in 10 minutes.
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17
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18
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CA 02239~07 1998-06-04
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19
The starting materials used were as follows:
(1) D9052: Ethylene-a-olefin copolymer/Softlex D9052
(manufactured by NIPPON OIL CO., LTD.),
density: 0.905 g/cm3
~2) P-0480: Ethylene-~-ole~in copolymer/Tafmer P-0480
(manufactured by Mitsui Petrochemical Co.,
Ltd.), density: 0.87 g/cm3
(3) Magnesium hydroxide: Kisuma 5B (manufactured by
Kyowa Kagaku K. K.), oleic acid-treated
product
(4) ~uicklime (1): Average particle size: 3 ~m, stea-
ric acid-treated product
(5) Quicklime (2): Average particle size: 10 ~m, stearic
acid treated product
(6) DBTDL: Dibutyltin dilaurate
(7) Antioxidant: Phenol type antioxidant/Irganox 1010
(manu~actured by Ciba Geigy)
(8) Lubricant: Low-molecular weight polyethyl-
ene/Sunwax 171P(manu~actured by Sanyo Kasei
Kogyo K.K.)
~9) EEA: Ethylene-ethyl acrylate copolymer (EA con-
tent: 23% by weight, crystalline melting
point: 93~C)
(10) SEPS: Hydrogenated styrene-isoprene block copolymer
(styrene content: 30% by weight, crystal-
line melting point: 135~C)
~11) ~-LDPE: Straight chain low-density polyethylene
(density: 0.924 g/cm3, MI: 3.0 g/10 min)
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(12) VTMOS: Vinyltrimethoxysilane
(13) DCP: Dicumyl peroxide
The evaluation methods were as follows:
(14) Amoun~ of water attached (ppm): Karl Fischer
method (150~C, 30 minutes)
~ (15) Silane impregnability: VTMOS/DCP liquid mixture was
heated with stirring in a super mixer and the
impregnation degree at that time was evalua-
ted.
o: Well impregnated, x: Impossible to impregnate
(16) Extruded tape appearance: 50 mm~ extruder, 120-
150-170-180-170~C, L/D: 20, Compression
ratio: 3.5, Tape die: 100 mm in width, 1 mmt
in lip clearance
Evaluation: o > ~ > x and the level of o was deter-
mined as "pass".
(17) Oxygen index: According to JIS K 7201
(18) Gel fraction (%): 120~C, 20 hours, xylene immer-
sion method
(19) Tensile strength (MPa) and elongation (%):
According to JIS K 6760
(20) Hot set: According to IEC-540A
o: Pass, ~: Failure
The flame-retardant polyolefin obt~;n~ and
the carrier polymer obt~ n~.rl were mixed in the ratio
shown in Tables 3 and 4 and the mixture was extruded
through an extruder and then immersed in a warm water to
subject the mixture to crosslinking treatment. The
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21
resulting extruded tape was used to evaluate gel frac-
tion, tensile strength, elongation and hot set. More-
over, the extruded tape was pressed to be subjected to
evaluation of oxygen index.
CA 02239507 1998-06-04
W O 97/24401 22 PCT/JP96103734
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23
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WO97/24401 PCT/JP96/03734
24
As is clear from Table 3 and Table 4, in Exam-
ples l to 4, the extrusion processability was good and
the crosslinking characteristics, m~r~h~n;cal characteris-
tics, flame-retardancy and heat resistance were excel-
5 lent.
On the other hand, in all the Comparative
Examples, extrusion processability, crosslinking charac-
teristics, mechanical characteristics, f~lame-retardancy
and heat resistance and the like were unbalanced.
lO Industry Applicability
According to this invention, it is possible to
obtain a flame-retardant, silane-crosslinked polyolefin
excellent in extrusion processability, and also excellent
in crosslinking characteristics, m~c-h~n;cal characteris-
15 tics, flame-retardancy and heat resistance.
