CA2076249A1 - Silane - cross linkable copolymer composition - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A silane-crosslinkable copolymer composition, comprising 100 parts by weight of a copolymer 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, v i n y l t r i e t h o x y s i l a n e a n d 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.%; from 0.001 to 10 parts by weight of a silanol condensation catalyst; and from 0.01 to 5 parts by weight of n-octyl triethoxysilane. The composition is of use in the manufacture of electric power cable insulation, pipes and moldings. The odourless composition has enhanced storage stability over similar compositions not having n-octyl triethoxysilane additive.

Description

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COMPOSITION

Field of the Invention The present inYen~ion relates to a silane-cr~sslinkable copolymer composition for use in various molding fields, and more particularly for electric power cable insulation.

Back~round of the Invention .
A method of silane-crosslinking a molded product of an ethylene copolymer graft-modified with an unsaturated silane compound, using water is known as is described in, for example, United Stat~s Patent No. 4689369, iss~ed August 25, 1987, to Mitsubishi Petrochemical Co., Ltd.
The silane-crosslinking method described in USP 4689369 is of industrial and commercial value in being extensively used in various fields, such as electric power cables, pipes, tubes, films, sheets, hollow moldings and foamed moldings.
However, in molding and crosslinking by silane-crosslinking using the ethylene copolymer obtained by radical polymerization of ethylene and unsaturated silane compound~, ¢ondensation reaction occurs at the initial , ' ' ' '.' ' ' ' . ", ' '. . ' ": ' ' ~', ' ' ''." ' ': . ' ,, :,, "' " ' ~ ' ' ' ' ~ , ' ~ ' ' ' ' : , , . , : ' ' . , '' .: ' '' :' . ' ~. ' , :.

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stage in an extruder during extrusion molding and unevenness tands to occur on the surface of a molding.
This defect is termed "scorching" and deteriorates the commercial value of the product, and improvement thereof is highly sought.
One solution to the problem of scorching is provided in USP 4689369, in which use of a scorch retardant to reduce scorching, while not decreasing the rate of crosslinking in the initial stage, is described. The scorch retardant is selected from a silane compound having an organic group, hydrolyzable during extrusion, and present in a specified amount in the silane-crosslinkable ethylene copolymer with a silanol condensation catalyst. The scorch retardant described in USP 4689369 may be a saturated or unsaturated silane, provided it has a hydrolyzable organic group. Examples of preferred scorch retardant silane compounds are vinyltrimethoxysilane, vinyltriethoxysilane and methacryloxypropyltrimethoxysilane.
However, in practising the invention disclosed in the USP 46~9369, several deficiencies become apparent.
With the hydrolyzable silane compound scorch retardants cited in USP 4689369 as preferred additives, specifically, vinyltrimethoxysilane and vinyltriethoxysilane, it has been found that under normal storage conditions the vapour pressure of these retardants is unsatisfactorily high. This results in a serious environmental and safety problem when storage packages containing the compositions, sealed against water vapour ingress, are opened to emit unpleasant odours, which in unfavourable circumstances may exceed allowable toxicity limits prior to adequate venting.
A second and related disadvantage is that when copolymer composition~ containing the defined scorch retardants are shipped or stored in bulk containers, such ~ , -. .
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~ 3 ~ SL250 as railcars, storage silos and the like, in which air is present in the headspace volume, the vapour concentration of the retardants may reach explosive levels.
A third and unfavourable e¢onomic disadvantage resulting from high vapour pressures of the retardants is that most of the benefit of the added retardant may be lost as the additive partitions to the gas phase over a period of time during the bulk storage. This leaves the copolymer deficient of the retardant component when the copolymer is ultimately extrusion molded.

Summary of the Invention Surprisingly, I have now found that the high vapour -pressure disadvantages encountered with the compositions described in USP 4689369, can be satisfactorily overcome without decreasing the rate of crosslinking of the copolymer composition in its initial stage, if an efficacious amount of a-octyltriethoxysilane i~ added as a scorch retardant to a silane crosslinkable e~hylene copolymer to which a silanol condensation catalyst has been added.
It is an object of the present invention to provide a silane-crosslinkable copolymer eomposition of sufficiently low vapour pressure as to be virtually odourless.
It is a further object of the invention to provide a silane-crosslinkable copolymer composition having no measurable vapour concentration when packages and the like are opened.
Yet further, it is an object of this invention to provide silane-crosslinkable copolymer compositions o~
enhanced storage stability.
Accordingly, the invention provides a silane-crosslinkable copolymer composition comprising: 100 parts ; ' ., ' . ,, ' ., , , , ~ ' - ' . :' , . : , : :
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by weight of a copolymer 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 apressure ranging from 1000 to 4000 kg/cm2, and containing said silane compound in an amount of from 0.5 to 2 wt.%; from 0.001 to 10 parts by weight of a silanol condensation catalyst; and from o.Ol to 5 parts by weight of n-octyl triethoxysilane.
Preferably, the ethylenically unsaturated silane compound is vinyltrimethoxy silane.
The invention, thus, provides a silane-crosslinkable copolymer composition comprising an additive which confers storage stability to the copolymer, in that not only does the specific silane retardant of use in the present invention overcome extrusion defects, but that when contacted with water vapour, the silane preferentially reacts with the moisture to prevent premature cross-linking of the copolymer. This not only prevents defective extrusion, but ensures consistent manufacturing process control with product batches, notwithstanding these batches may have been stored over various periods oP time.

Detailed_Description of the Invention The ethylene silane-crosslinkable copol~mer of use in the composition of the present invention is a copolymer 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 - , . .
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wt% of copolymerizable monomers other than ethylene and the ethylenically unsaturated silane compound having a hydroly2able organic group. Examples of such optional monomers include a-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 a-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 blocX form.
The ethylenically unsaturated silane compound which can be used is selected from various compounds having an ethylenically unsaturated bond copolymerizable with ethylene, and a hydrolyzable silane group. These compounds are represented by the formula:
RSiR~Y3-n wherein R is an ethylenically unsaturated hydrocarbyl or hydrocarbyloxy group; R' is an aliphatic saturated hydrocarbyl group; Y which is the same or different is a hydrolyzable organic group; and n is 0, 1 or 2.
Examples of the unsaturated silane compounds are the compounds of the above-described formula wherein R is vinyl, allyl, isopropenyl, butenyl, cyclohexenyl or Y
methacryloxypropyl; Y is methoxy, ethoxy, formyloxy, acetoxy, propionoxy, alkyl or arylamino; and R' is methyl, ethyl, propyl, decyl or phenyl.
The particularly preferred unsaturated silane compounds are compounds represented by the following formula, and ~-methacryloxypropyltrimethoxysilane:
CH2=CHSi(OA~3 wherein A is a hydrocarbyl group having 1 to 8, preferably 1 to 4, carbon atoms.

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The most preferred unsaturated silane compounds are vinyltrimethoxysilane, vinyltriethoxysilane and ~ -methacryloxypropyltrimethoxysilane.
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/cm2, 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 about 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 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 lauroyl peroxide, ~-dipropionyl peroxide, benzoyl peroxide, di-t-butyl peroxide, t-butyl hydroperoxide, and t-butyl peroxyisobutyrate; tb) molecular oxygen; and (c) azo compounds such as azobisisobutyronitrile and azoisobutylvaleronitrile.
Examples of the optional comonomers are the same as the above described copolymerizable monomers.
Examples of the chain transfer agent include (a) paraffinic hydrocarbons such as methane, ethane, propane, butane and pentane; (b) a-olefins such as propylene, butene-1 and hexene-1; (c) aldehydes such as formaldehyde, acetaldehyde and n-butylaldehyde; (d) ,' ' .
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2 ~ 3 ketones such as acetone, methyl ethyl ketone and cyclohexanone; (e~ aromatic hydrocarbons; and (f) chlorinated hydrocarbons.
The copolymer used in the composition of the present invention contains 0.1 to 5 wt%, preferably 0.3 to 3 wt%, and more preferably 0.5 to 2 wt%, of the unsaturated silane compound unit.
The higher the content of the unsaturated silane compound in the copolymer, the greater the mechanical strength and heat resistance of the silane-crosslinked product thereof. However, if the content of such unsaturated silane compound is excessively high, the t nsile elongation and heat sealability of the crosslinked product are reduced. In view of this, the content of the unsaturated silane compound in the copolymer is limited to the range of 0.1 to 5 wt%. This copolymer can be blended with other olefinic polymers, and even in this case, the content of the unsaturated silane compound in the blend must be limited to the range of 0.1 to 5 wt%.
The silanol condensation catalyst used in the ~-compositisn of the present invention is generally selected from the compounds which can be conventionally used as a catalyst for accelerating dehydration condensation between silanol groups.
Examples o~ the silanol condensation catalysts are carboxylic acid salts of metal such as tin, zinz, iron, lead and cobalt, organic bases, inorganic acids, and organic acids.
Representative examples of the silanol condensation catalysts are (1) carboxylic acids of metals such a~
dibutyltin dilaurate, dibutyltin diacetate, dibutyltin dioctoate, stannous acetate, stannous caprylate, lead naphthenate, lead caprylate and cobalt naphthenate; (2) organic bases such as ethylamine, dibutylamine, ', ' .' ' ' ' ' . ' ,, ' '. . ' " " ~, . ~ , , ~' '' ' . , : ' ' . ' ' : ' , " ,., ' , . . . ' hexylamine and pyridine; (3) inorganic acids such as sulfuric acid and hydrochloric acid; and (4) organic acids such as toluenesulfonic acid, acetic acid, stearic acid and maleic acid.
The silanol condensation catalyst is used in an amount of 0.001 to 10 parts, preferably 0.01 to 5 parts, and more preferably 0.01 to 3 parts, by weight per 100 parts by weight of the silane-crosslinkable ethylene copolymer. If the amount of the silanol condensation catalyst is less than 0.001 part by weight per 100 parts by weight of ethylene copolymer, the cross-linking reaction does not proceed sufficiently. If, on the other hand, the amount of the silanol condensation catalyst is ~ -more than 10 parts by weight per 100 parts by weight of local gelation proceeds in the extruder during extrusion and the extrudate has a very poor appearance.
n-Octyltriethoxysilane is used in an amount of 0.01 to 5 parts, preferably 0.05 to 3 parts, and more preferably 0.1 to 2 parts, by weight per 100 parts by weight of the ethylene copolymer. If the amount of n-octyltriethoxysilane is less than 0.01 part by weight per 100 parts by weight of the ethylene copol~mer, the desired effect of the present invention cannot be obtained. If, on the other hand, the amount of n-octyltriethoxysilane is more than 5 parts by weight per 100 parts by weight of the ethylene copolymer~ the rate of crosslinking at its initial stage decreases csnsiderably and, in order to obtain a product having a sufficient heat resistance, the crosslinking treatment time increase~, resulting in deterioration of operation efficiency.
The composition of the present invention is sufficient if it has the above-described compositions prior to kneading. For example, the three ingredients of the invention as hereinabove defined may ~e prepared into ,. . .. ~ . - , . . ~ .

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, the desired composition in a k~eader. Kneading can be conducted by conventional methods. Use of an extruder is preferred. The kneaded product is then silane-crosslinked with water for use, for example, as electric cable insulation.
The composition of the present invention shows the expected remarkable effect as described in USP 4689369, in that the appearance of the molded product obtained herein is improved by the simple manner of adding a suitable amount of the octyltriethoxysilane compound as at extrusion kneading. This greatly increases the practical value of the composition of the present invention as a molding material for a resin molding.
The following examples and comparative examples are provided to further illustrate the composition of the present invention, but are by no means intended as limiting.

EXAMPLES
A commercially produced under high pressure, free radical copolymer of ethylene and vinyl trimethoxy silane in pellet form maintained dry in water impermeable packaging, and sold under the trademark AQUA-LINX~ (AT
Plastics Inc., Ontario, Canada), was selected as a control (sample 1) in the following experiments. The copolymer had the following characteristics:-Melt Index 0.8 ~
Silane fraction 1.8% W/W -6.0 g of vinyltrimethoxysilane was added to 2000 g of the control copolymer in a foil-lined polyethylene pouch, which was immediately sealed closed by means of a heat sealer. The pouch and contents were thoroughly agitated and then placed in an oven with temperature controlled to 50C at which condition, the silane additive infused into the pellets. This was labelled sample 2. Using the identical process, 6.0 g of n-octyltriethoxysilane (OCTEO) was added to 2000 g of the controlled to become sample 3. Samples 1, 2 and 3 were used in the test for Improved Storage Stability.

Test for Improved Storaqe Stability The improved storage stability conferred upon the ethylene vinyl silane copolymer by the n-octyltriethoxysilane was verified by an accelerated a~ing procedure in which the melt index of the copolymer was monitored as a function of aging time at so~C and 100%
relative humidity (RH). Table 1 gives % retention of melt index of the copolymer with and without free silanes, namely, vinyl trimethoxysilane (VTMOS) and octyltriethoxysilane (OCTEO) 0.3%W/was additives.

Aain~ Conditions - 90C. 100% RH

Samples % Retention of melt index after 1 Day 4 Days 10 Days 1. EVS Copolymer 52 62 0 2. EVS Copolymer ~ ~.
0.3~ VTMOS 73 47 0 3. EVS Copolymer ~
0.3% OCTEO 94 51 0 The results in Table 1 show that while both VTMOS
and OCTEO preferentially react with moisture preventing the molecular enlargement which is manifested as a better retention of melt index than the EVS copolymer sample without these said additives, the real advantage of OCTEO

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~ SL250 over VTMOS stems from the difference in saturated vapour pressure. The lower vapour pressure o~ OCTEO permits it to stay in the copolymer for longer periods of time at a given temperature. This results in a higher percentage of retention of melt index for a given time.

Test for Improved Process Stability Additional samples at 0.5% vinyltrimethoxysilane and 0.5% n-octyltriethoxysilane were produced using the techni~ue as described in Example I. To 100 parts by weight of each o~ these and to the control copolymer, was added 5 parts by weight of a catalyst masterbatch composed of polyethylene containing 1~ by weight of dibutyltin dilaurate. These became samples 4, 5 and 6 and were used in the Test ~or Improved Process Stability.
The samples were melted and mixed in a Brabender chambar with a speed of 20 RPM for the mixing heads. The mixing was done at three different tempPratures and the torque was monitored as a function of mixing time. The actual melt temperature was measured ac stock temperature which was 10C lower than the chamber control temperature.
The time to reach the minimum torque, the minimum torque itself and the rate of its increase are considered to be the important process parameters; these are measured and reported in Table 3.

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Table 2 ~xamples Sample No. Composition 4 EVS copol~mer + 5% Catalyst Masterbatch EVS copolymer + 0.5% free VTMOS + 5%
Catalyst Masterbatch .
6 EVS copolymer + O.5~ free OCTEO + 5 Catalyst Masterbatch - . , . . , . . . , : .. : , . . . ... : ~ ., . : . . .,, . . . : . . . :
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~ SL250 The lowest rate of increase of torque corresponds to the copolymer with ~CTEO as crosslinking stabiliser for all three test temperatures. This is of practical significance. As the polymer becomes more insensitive to the residence time in an extruder, it facilitates tool change and interruptions in the production line without the necessity of stopping and cleaning the extruder, and the surface finish of the extruded product can recover to its excellence ~ithin a short time.

The minimum torque and the time to reach this value are indicative of how easily and how quickly the steady state can be reached while extruding a compound.
Comparing the values given in Table 2, it is seen that when VTMOS was used as an additive, lowest torque values are reached. Because of its lower molecular weight it may act as a plasticiser in reducing the minimum torque.
This specific advantage is off-set by its higher rate of tor~ue increase, which makes VTMOS less efficient than OCTEO for process stability.

Amount Produced: 5.8kgJhr While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent *o one skilled in the art that various changes and modification can be made therein without departing from the spirit and scope thereof.

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Claims (2)

1. A silane-crosslinkable copolymer composition, comprising:
100 parts by weight of a copolymer 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, v i n y l t r i e t h o x y s i l a n e a n d 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.%; from 0.001 to 10 parts by weight of a silanol condensation catalyst;
and from 0.01 to 5 parts by weight of n-octyl triethoxysilane.

2. A composition as claimed in claim 1 wherein said ethylenically unsaturated silane compound is vinyltrimethoxysilane.