Note: Descriptions are shown in the official language in which they were submitted.
` 208~36 ~
~ ,~
- ~,-
- 1 - SL257
ILANE-CROSSLINXABLE COPO~YMER COMPO8I~IONS
FIELD OF THE INVEN~ION
This invention relates to a silane crosslinkable
copolymer composition for use in various molding fields
and, particularly, for cable insulation.
BACl~GROUND OF TXE IN~tENTION
Polyethylene polymer such as low density
polyethylene and the like is generally crosslinked to
enhance mechanical strength and heat resistance.
Crosslinking is generally carried out by either peroxide
crosslinking, radiation crosslinking or moisture
crosslinking. In the latter crosslinking method, for
example, a hydrolyzable silane group is introduced into ~ ~ ;
the polyethylene, which in turn is used as the
crosslinkable group. The hydrolyzable silane group i5
introduced either by grafting or copolymerization.
Examples of relevant prior art references are United
States Patent No. 4,297,310, 4,351,876, 4,397,981,
4,413,066 and 4,689,369 and U.K. Patent 1,581,041. In ;~
most of these references a vinyl alkoxysilane such as
vinyltrimethoxy silane or vinyltriethoxy silane has been
used. The silane cross-linking method described in ~
~ :'
: .
~84361
- 2 - SL257
aforesaid USP 4689369 is of industrial and commercial
value in producing compositions extensively used in
various fields, such as electric cables, pipes, tubes,
films, sheets, hollow moldings and formed moldings.
A major drawback in the manufacturing of these
compositions involving hydrolyzable silane groups grafted
or copolymerized with polyethylene polymers is that the
resultant composition or product has relatively low
crosslinking rates and, thus, require a long time under
ambient conditions to cure. Even in water or steam at
higher temperatures, the curing time is significantly
long. It has long been recognized that a reduction in
the curing time would provide economic advantages to the
end producer using such crosslinkable compositions.
Further, if curing could be effected at ambient
temperature this would result in a reduction in capital
equipment costs.
U.S. patent No. 4446283 describes the room
temperature crosslinking of a specific class of
alkoxysilane ethylenic esters. Similar room temperature
crosslinking has been achieved by qrafting vinyl
alkoxysilane into more amorphous ethylene propylene
rubbers ("Silane Grafted Ethylenepropylene Elastomers for
the Cable Industry", S.Cartasegna, Rubber Chem
Technology, vol. 59 p. 722-739, 1986).
SUMMARY OF ~HE INVENTION
Surprisingly, I have now found that crosslinking of
hydrolyzable silane groups comprising copolymers of
ethylene and unsaturated silane compounds can be
satisfactory crosslinked at ambient temperatures, or at
higher temperatures more rapidly than is done presently,
if an efficacious amount of a polyhydroxy additive
compatible with the grafted or copolymerized silane
~3
20~43~,
_ 3 - SL257
copolymer is present.
It is an object of the present invention to provide
a silane crosslinkable copolymer composition
crosslinkable under ambient conditions.
It is a further object of the invention to provide
a silane crosslinked polyethylene copolymer composition
in a relatively shorter period of manufacturing time than
is conventionally known in the art.
Accordingly, the invention in one aspect provides a
silane crosslinkable copolymer composition comprising a
silane crosslinkable copolymer of the general formula:
~CH2 CH2)x (COM)y
,
wherein COM is:
( CH2 CR
~ ,.
2 0 ( CH2 ) ~
(CHR2)n : :
Z",
,'';~
Si (oR3~ 3 :
and Z is -CO-O-CH2CH2-; M is O or 1; n is 0 or l;
Rl=R2 is H or CH3; R3 is CH3 or C2Hs; and x is greater than
50 and y is 0.5 < (MW~o~ x y x 100 < 6.0; - 1
(28 x x + (~qwco~) x y)
and MWC~ is the molecular weight of the comonomer; and an
efficacious amount of a polyhydroxy additive compatible
with said copolymer.
Preferably the copolymer is of the general formula -
selected from the group consisting of:
~:
: :
2~8~36,.
_ 4 - SL257
~CH2 CH2)x(cH2 C Rl~y
I
Si(oR2~3
and ~CH2 CH2)X (CH2 C R~y
CO. O(CH2)3 Si(o C~3)3
where Rl is H or CH3 and R2 is CH3 or CzH5;
and wherein said copolymer is prepared by radically
polymerizing a polymerizable monomeric mixture consisting
essentially of ethylene and at least one ethylenically
unsaturated silane compound selected from the group
consistingofvinyltrimethoxysilane,vinyltriethoxysilane
and methacryloxypropyltrimethoxysilane under a pressure
ranging from 1000 to 4000 kg/cm2, and containing said
silane compound in an amount of from 0.5 to 2 wt.%.
¦ 25 In an alternative preferment, the copolymer is agraft copolymer of the general formula selected from the
group consisting of:
2~36~
_ 5 - SL257
~CH2 CH2)X (CH2 CH~y
I
CH2
1 ~
CH2
. (R3 0~3 Si
wherein R3 is CH3 or C2H5;
and ~CH2 CH2~x (CH2 CH)
I .~::
CH2
1 ~
C-CH3 ~ :
I ' ~-
C=O ~ ~:
; 30 i :~ :
OCH2CH2CH2 Si(o CH3)3
','
Most preferably the ethylenically unsaturated silane .::-~
compound is vinyltrimethoxysilane which provides ethylene -
vinyltrimethoxysilane copolymer; and the copolymers are ~ :
generally known as ethylene-vinyl silane (EVS) ~ :~
copolymers.
The present invention is based on the surprising ~ ::
discovery that by the addition of an efficacious amount
of a polyhydroxy additive compatible to the crosslinkable
silane copolymer the rate of cross-linking is enhanced.
While not being limited by theory, it is believed that
this surprising effect is due to the acceleration of :
, 45 water absorption by the additive, particularly if the `:
additive has hygroscopic properties to some degree, but,
~.
b
` - 20~3~-i
- 6 - SL257
wherein the water absorbed facilitates the crosslinking
by hydrolysis of the silane groups in the copolymer.
The additives used in the practice of this invention
are selected from the general class of compounds having
substituents capable of hydrogen bonding with water.
Examples of these substituents are hydroxyl groups,
carboxyl groups, amine groups, and the like. Examples of
additives having these substituents are polyethylene
glycol, polysaccharides, rosins, polyols, fatty acids,
phenols and alcohols.
The efficacious and relative amounts of silane
crosslinkable copolymer and polyhydroxy additive in the
composition according to the invention may be readily
determined by the skilled person in the extrusion and
molding art. Typically, the additive constitutes 0.1-10%
w/w of the crosslinkable composition.
Thus, the copolymer can be in the form of a normal
copolymer of ethylene and unsaturated organosilane such
as vinylalkoxysilanes copolymerized under high pressure
using a tubular or autoclave reactor with any of the
known free radical initiators employed in olefin
polymerisation technology. It can be in the form of a
grafted copolymer prepared by graft polymerisation of an
unsaturated organo-silane onto polyethylene or copolymers
of ethylene or copolymers of ethylene, propylene and
other monomers. The copolymerised silane copolymer is
preferred from the viewpoint of its stability and
processability.
The copolymer described in the present invention
contains silane units in a quantity of 0.01 to 5% more
preferably 0.01 to 2% by weight.
Preferably, a catalyst to promote the condensation
reaction between two adjacent silanol groups is also used
in the present invention. In general, such silanol
condensation catalysts can be a carboxylate of a metal
-~ 208~3~.
_ 7 - SL257
such as tin, zinc, iron, lead and cobalt, an organic
base, an inorganic acid and an organic acid. Examples of
silanol condensation catalyst are dibutyl tin dilaurate
(DBTDL), dibutyl tin diacetate (DBTDA), dibutyl tin
dioctoate (DBTDO), dioctyl tin maleate (DOTM), stannous
acetate, stannous caprylate, lead naphthenate, zinc
caprylate, cobalt naphthenate, ethylamines,
dibutylamines, hexylamines, pyridine, inorganic acid and
organic acids such as toluene sulfonic acid, acetic acid,
stearic acid and maleic acid.
The amount of silanol condensation catalyst to be
used can be determined with reference to the examples
given below. It is generally 0.01 to 1% by weight of the
entire reaction material. Specifically, in the case of
organic tin catalyst the tin content should be in the
range of 50-500 ppm, preferably in the range 100 - 300
ppm of the entire reacting material.
The catalyst can be added to the copolymer by any
method that can be employed for incorporating additives
in thermosplastic resins. As the amount of the catalyst
to be added is small, it is convenient to prepare a
masterbatch wherein a high concentration of catalyst is
dispersed in a polymer medium such as polyethylene or an
ethylene vinyl acetate copolymer. The masterbatch can
then b~ added to the silane copolymer in such an amount
that a predetermined concentration of catalyst will be
present in the polymer.
The said masterbatch can contain a variety of
auxiliary materials such as stabilizers (anti-oxidants,
metal deactivators etc.), clay fillers, halogenated and
non-halogenated flame retarding fillers, synergists,
colouring agents, foaming agents, carbon black etc.
Additives such as polyethylene glycol of different
molecular weights, sorbitols and acetal and ketal
derivatives thereof, particularly dibenzylidene sorbitol,
/
-- 2~8~36 ~
- 8 - SL257
rosins and polyols are used along with the copolymer and
catalyst masterbatch in appropriate proportions to
enhance the water absorption required to convert the
silane groups in the polymer to silanol groups which are
then condensed in the presence of catalyst forming the
crosslinked network. Ethylene vinyl acetate (alcohol)
terpolymer, herein after referred to as EVA (OH)
terpolymer is also used in the practice of the invention.
The EVA (OH) terpolymer is obtained by hydrolysis of
ethylene-vinyl acetate copolymer in solution, emulsion or
suspension and in a batch process or in a reactive
extrusion as a continuous process. The acetate ester
group must be effectively hydrolyzed to 25-95%, under
hydrolysis conditions known to the art. The amount of
EVA (OH) terpolymer used in the present invention is
determined with reference to target cure time.
Alternately, the time for crosslinking the silane
copolymer is controlled by the amount of these
additives/terpolymer limiting the ingress of moisture by
the extruded and/or moulded products during and after
cooling in water.
The additive component to increase the moisture
absorption can be added either directly or can be
compounded into the catalyst masterbatch in appropriate
proportions.
In a further aspect, the invention provides a method
of crosslinking a silane crosslinkable copolymer of the
general formula as set forth hereinabove when present in
the composition as set forth hereinabove, said method
comprising treating said composition with water at
ambient temperature to provide said crosslinked
composition.
In yet a further aspect, the invention provides a
crosslinked copolymer composition produced by the method
as hereinbefore defined. The crosslinked copolymeric
} ::
208~3~1 ~
- 9 - SL257
:~ :
composition is of particular value when used as an
electrical insulation coating for electric cables.
: ~:
DE~AILED DESCRIPTION OF THE INVENTION ~ ~ ~
-: :
The ethylene silane-crosslinkable copolymers of use
in the compositions of the present invention are
copolymers consisting essentially of ethylene and an
ethylenically unsaturated silane compound having a
hydrolyzable organic group.
The term "consisting essentially of" used herein
means that the ethylene copolymer can contain up to 30 ~ -
wt% of copolymerizable monomers other than ethylene and
the ethylenically unsaturated silane compound having a
hydrolyzable organic group. Examples of such optional
monomers include -olefins such as propylene, hexane-1
and 4-methylpentene-1; vinyl esters such as vinyl acetate
and vinyl butyrate; unsaturated organic acid derivatives
such as methyl acrylate, ethyl acrylate and methyl
methacrylate; unsaturated aromatic monomers such as
styrene and -methylstyrene; and vinyl ethers such as
vinylmethyl ether and vinylphenyl ether. These optional
monomers can be present in the ethylene copolymer in any~
forms, e.g. a graft form, a random form or a block form.
Ethylene and the unsaturated silane compound are
copolymerized under any conditions such that ~-
copolymerization of the two monomers occur. More
specifically, those monomers are copolymerized under a
pressure of 500 to 10,000 kg/cm2, preferably 1,000 to
4,000 kg/cm, and at a temperature of 100 to 400C.,
preferably 150 to 350C., in the presence of a radical
polymerization initiator, optionally together with up to
abo~t 40 wt% of a comonomer and a chain transfer agent.
The two monomers are brought into contact with each other
simultaneously or stepwise in a vessel or tube type
,
,,",,",,~,",,,"~~ ,"",,~.,",,,,,.",~,,.,"j,"-"~,",,,~
'" 2~3~
- 10 - SL257
reactor, preferably in a vessel type reactor.
In the copolymerization of ethylene and the
unsaturated silane compound, any radical polymerization
initiators, comonomers and chain transfer agents, which
are conventionally used in homopolymerization of ethylene
or copolymerization of ethylene with other monomers can
be used.
Examples of radical polymerization initiators
include (a) organic peroxides such as lauryl peroxide,
dipropionyl peroxide, benzoyl peroxide, di-t-butyl
peroxide, t-butyl hydroperoxide, and t-butyl
peroxyisobutyrate; (b) molecular oxygen; and (c) azo
compounds such as azobisisobutyronitrile and azo
isobutylvaleronitrile.
Examples of the chain transfer agent include (a)
paraffinic hydrocarbons such as methane, ethane, propane,
butane and pentane; (b) -olefins such as propylene,
butene-1 and hexene-1; (c) aldehydes such as
formaldehyde, acetaldehyde and n-butylaldehyde; (d)
¦ 20 ketones such as acetone, methyl ethyl ketone and
cyclohexanone; (e) aromatic hydrocarbons; and (f)
chlorinated hydrocarbons.
~ While the copolymer of use in the present invention
I can be in the form of a normal copolymer of ethylene and
unsaturated organosilane copolymerized under high
pressure using a tubular or autoclave reactor with free
radical initiators as hereinabove described, the
copolymer can also be of the form of a graft copolymer
prepared by graft polymerization of an unsaturated organo
silane onto polyethylene or copolymers of ethylene or
copolymers of ethylene and other monomers. Methods of
making such graft copolymers are known in the art.
The ingredients of the invention as hereinabove defined
may be prepared into the desired composition in a
¦ 35 kneader. Kneading can be conducted by conventional
, 208~36~
:
- 11 - SL257
methods. Use of an extruder is preferred. The kneaded
product is then silane-crosslinked with water for use,
for example, as electric cables, pipes, films, foamed
products, and the like.
The following description and examples are provided
to further illustrate the compositions of the present
invention, but are by no means intended to be limiting.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS . :
A commercially produced (under high pressure, free
radical polymerisation) EVS copolymer of ethylene and
vinyl trimethoxy silane in pellet form maintained dry in
water-impermeable packaging, and sold under the trade-
mark AQUA-LINX ~ (AT Plastics Inc., Ontario, Canada),
was used in the following experiments.
Further in the examples cited below, evaluation of
the compositions was performed while coating a 14 AWG
wire by means of an extruder, to a thickness of 1 - 1.5
mm. The coated wire samples were kept at ambient
temperature and humidity conditions (typically 22-25C at
45-55% ~H). Hot creep elongation on the coatings was
measured as per procedure ICEA T-28-562 at different
intervals of time. Hot creep elongation was used as a
measure of crosslinking. This is a simple and quick
procedure, as opposed to the gel measurement, to judge
crosslinking. The relationship between gel and hot creep
elongation was also established to serve as a guideline
for this evaluation.
I have also found that the selected polyhydroxy
additive can be added separately prior to extrusion
molding without detracting from the storage stability of
the silane crosslinkable copolymer.
:.
3 ~ ~
- 12 - SL257
Example l
To an ethylene vinyl trimethoxy silane (EVTMOS)
copolymer was added 5% by weight of a masterbatch
containing 1% by weight of dibutyl tin dilaurate (DBTDL)
and a necessary amount of a phenolic ester anti-oxidant
and a metal deactivator. The blend of copolymer and
masterbatch was extruded onto a 14 AWG copper conductor
to a thickness of 0.75 - l.Omm. The insulated wire was
allowed to stand in an atmosphere of 23C/45 - 55%
relative humidity or immersed in hot water at 90C. The
percentage of gel and the hot creep elongation were
measured at different cure times. It was found that hot
creep elongation follows a linear relationship with gel
(Table 1).
Table 1
95% EVTMOS copolymer and 5% masterbatch
containing 1% DBTDL
% Hot Creep Elongation % Gel
at 150C/0.20MPa/15min
7~ 62
100 57
125 48
Hot creep elongation of 75-50% corresponding to a gel of
62-70% was chosen as a benchmark for complete
crosslinking to yield a product with very good thermal
and mechanical properties. The time to reach this range
of value for creep elongation, preferably 100%, with the
samples maintained at ambient temperature and humidity
conditions, may then be considered as a measure of the
crosslinking speed of different compositions.
2~36~
- 13 - SL257
ExamPles 2, 3, 4, 5
These examples are chosen to demonstrate the influence of
effective tin concentrations in the entire reacting -~
material on the cure time as evaluated by the
measurements of creep elongation. DBTDL, dibutyl tin
diacetate (DBTDA) and DOTM were used for these studies.
Table 2 gives the details of the catalyst concentrations
in the masterbatches.
Table 2
_ _
Ex.No. Catalyst % by wt in ppm of tin in
Ma~terbatch EVTMOS + 5%
Masterbatch
2 DBTDL 1 90
I
3 DBTDA 1 180
I . I ,,
¦ 4 DOTM 1.45 180
DOTM 2.18 270
.
The wire specimens coated with silane copolymer and 5% by
weight of these masterbatches were allowed to stand at
23C/45-55% relative humidity. The creep elongation at
150C was measured at different cure times as represented
in Table 3.
Table 3
_
Cure Time % Hot Creep Elongation at ¦
(Days) 150C/0.2OMPA/15 min
Ex.2 Ex.3 Ex.4 Ex.5 ¦ ~ ;
I ..
4 190 150 137 112 ~ -
I
1 8 146 125 87 90 ~
1 10 133 111 75 _75
::
2 ~
- 14 - SL257
As the concentration of tin in the entire reacting
material is changed from 90 to 270 ppm, the time ts reach
100~ hot creep elongation is shortened. These examples
demonstrate an improvement in the speed of crosslinking
silane copolymers as achieved by controlling the amount
of tin needed for the catalytic condensation of
neighbouring silanol groups.
EXAMPLES 3, 6, 7, 8, 9
Whereas Examples 2, 3, 4 and 5 have addressed the
condensation of silanol groups, Examples 6, 7, 8 and 9
focus on the conversion of silane groups to silanol
groups by increased water absorption. DBTDA masterbatch
containing 1% by weight of the catalyst tExample 3) is
used for this objective. Dibenzylidene sorbitol (DBS),
polyethylene glycol (PEG), and EVA(OH terpolymer with 99% -~
and 40% hydrolysis were aded at appropriate levels to the
blend of the silane copolymer and 5% catalyst masterbatch
and extruded onto wires. The results obtained as per
procedure in previous examples are reported in Table 4. ~-
~able 4
_ _
Cure Time % ~ot Creep Elongation at
~Days) 150C/0.20~PA/15 ~in
Ex.3 Ex.6 Ex.7 Ex.8 Ex.9 --
4 150 100 125 97 62
1~5 75 100 75 51
111 62 96 62 50
_
2~8~
- 15 - SL257
* Example 3: 95% w/w EVTMOS Copolymer + 5% w/w DBTDA
ma~terbatch
* Example 6: " " + 0.5% w/w DBS
* Example 7: " " + 0.25% w/w PEG
* Example 8: Example 3 + 5% w/w EVA(OH) (40% hydrolysed)
Example 9: Example 3 + 5% w/w EVA(OH) (99% hydrolysed)
In these examples with a lower active tin concentration
in the entire reactive material, shorter time to full
crosslinking is achieved by adding appropriate amount of
compounds with hydroxyl groups.
Example 10
In examples 6, 7, 8 and 9 the component to increase water
absorption is added separately. In Example 10, DBS was
added to the catalyst masterbatch containing DOTM such
that the final crosslinkable compound with 5% by wt of
this masterbatch contained 180 ppm of tin and 0.5% by wt
of DBS. The crosslinking of this composition at ambient
conditions was compared to Example 5 containing 270 ppm
of tin (DOTM) and no DBS. (Table 5); and crosslinking in
water at 90C was compared to Example 2 containing 90ppm
of tin (Table 6)
i
2~8~3~
- 16 - SL257
~able_5
_ _
Cure Time% Hot Creep ~longation at
(Days)150C/0.20MPa/15 min
Example 5 Example 10
112 ~62
I
8 90 55
I
. I
Table 6
_
Cure Time% Hot Cre~p Elongation at ¦
(hrs)lS0C/0.2OMPa/15 min ¦
I
Example 2 Example 10 ~
, ,-.
: 1 150 75
110 55
4 75 50
48
_ _ . _ _
The present discovery therefore brings out the importance
of the synergism between silanol conversion and catalyst
condensation to form the crosslinked network in a shorter
time with the material still capable of being formed or
extruded into a product. The method indicated in the
present invention leads to a composition that can be
crosslinked either at room temperature, or at higher
temperature within a shorter interval of time compared to
: :
'
2~36 ~
- 17 - SL257
conventional composition of current industrial practice;
more specifically to a composition containing a tin
catalyst with a preferred concentration of tin at 120-200
ppm and an appropriate amount of component to increase
water absorption, preferably a chemical component with
hydroxyl groups.
The present invention also offers a means to reduce
the time required to cure power cables with thicker
insulations. In addition the polyhydroxy compounds used
in the present invention are expected to minimize the
micro condensation of water and hence retard the
propagation of water trees, rendering a long life time to
the underground power cables.
While the invention has been described in detail and
with reference to specific embodiments thereof, it will
be apparent to one skilled in the art that various
changes and modifications can be made therein without
- departing from the spirit and scope thereof.
