CA2084361A1

CA2084361A1 - Silane-crosslinkable copolymer compositions

Info

Publication number: CA2084361A1
Application number: CA 2084361
Authority: CA
Inventors: Haridoss Sarma
Original assignee: AT Plastics Inc
Current assignee: AT Plastics Inc
Priority date: 1992-12-02
Filing date: 1992-12-02
Publication date: 1994-06-03

Abstract

ABSTRACT OF THE DISCLOSURE

A composition comprising a silane crosslinkable copolymer of the general formula: A composition comprising a silane crosslinkable copolymer of the general formula: ?CH2 CH2)x(COM)y wherein COM is:

and Z is -CO-O-CH2CH2-; M is 0 or 1; n is 0 or 1;
R1=R2 is H or CH3; R3 is CH3 or C2H5; and x is greater than 50 and y is 5.0 ? ? 6.0;
and MWcom is the molecular weight of the comonomer; and an efficacious amount of a polyhydroxy additive compatible with said copolymer. The copolymers can be readily crosslinked at ambient temperatures or more rapidly at higher temperatures. The compositions are of use in various molding fields and, particularly, electric cable insulation.

Description

` 208~36 ~
~ ,~
- ~,-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 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

1 ~

. (R3 0~3 Si wherein R3 is CH3 or C2H5;
and ~CH2 CH2~x (CH2 CH) I .~::

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 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 ~

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 ~

:~ :
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~

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~

:

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 ~ ~

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 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~

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 I

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 ~

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 --1~5 75 100 75 51 _ 2~8~

* 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~

~able_5 _ _ Cure Time% Hot Creep ~longation at (Days)150C/0.20MPa/15 min Example 5 Example 10 112 ~62 I

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 _ _ . _ _ 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 ~

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.

Claims

1. A composition comprising a silane crosslinkable copolymer of the general formula:

?CH2 CH2)x(COM)y wherein COM is:

and Z is -CO-O-CH2CH2-; M is 0 or 1; n is 0 or 1;
R1=R2 is H or CH3; R3 is CH3 or C2H5; and x is greater than 50 and y is 0.5 ? ? 6.0;
and MWcom is the molecular weight of the comonomer; and an efficacious amount of a polyhydroxy additive compatible with said copolymer.

2. A composition as claimed in Claim 1 wherein said copolymer has the general formula selected from the group consisting of:

and ;

where R1 is H or CH3 and R2 is CH3 or C2H5;
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 consisting of vinyltrimethoxysilane, 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.%.

3. A composition as claimed in Claim 1 wherein said copolymer is a graft copolymer of the general formula selected from the group consisting of:

wherein R3 is CH3 or C2H5;
and

4. A composition as claimed in Claim 2 wherein said ethylenically unsaturated silane compound is vinyl trimethoxysilane.

5. A composition as claimed in claim 1 wherein said compatible polyhydroxy additive is selected from the group consisting of polyethylene glycols, sorbitol and acetal and ketal derivatives thereof, rosins, polyols, and ethylene vinyl acetate (alcohol) terpolymer.

6. A composition as claimed in claim 5 wherein said compatible polyhydroxy additive is selected from the group consisting of dibenzylidene sorbitol and ethylene vinyl acetate (alcohol) terpolymer.

7. A composition as claimed in claim 5 further comprising a tin catalyst at a concentration of 120-200 p.p.m. % W/W.

8. A method of crosslinking a silane crosslinkable copolymer of the general formula as set forth in claim 1 when present in the composition as claimed in claim 1, said method comprising treating said composition with water at ambient or higher temperatures to provide said crosslinked composition.

9. A method as claimed in claim 6 wherein said crosslinkable composition comprises a tin catalyst at a concentration of 120-200 p.p.m. W/W.

10. A molded product comprising a crosslinked ethylene vinyl silane copolymer produced by the method as defined in claim 8 or claim 9.

11. A product as claimed in claim 10 comprising an electrical conductor coated with said crosslinked ethylene vinyl silane copolymer.