AU463500B2

AU463500B2 - Cross-linking process

Info

Publication number: AU463500B2
Application number: AU60862/73A
Authority: AU
Inventors: George Scott Henry
Original assignee: Dow Corning Ltd
Current assignee: Dow Silicones UK Ltd
Priority date: 1972-10-28
Filing date: 1973-10-02
Publication date: 1975-07-31
Also published as: GB1396120A; JPS5120210B2; DE2353783A1; DE2353783B2; FR2204639A1; DE2353783C3; JPS49134751A; FR2204639B1; AU6086273A

Description

60,862/7 This invention relates to a method for cross-linking certain ethylenor-aeyl ecrylate copolymers and .is concerned with an improvement in or modification of the invention claimed in Application No. 65517/69. In Application No. 65517/69 there is described and claimed a process.for effecting the crossit1nking o a polyolefin which comprises reacting a polyolefin which is polyethylene or a copolymer of ethylene with a minor proportion of propylene and/or butylene, with a silane of the general formula RR'SiY 2 (wherein R represents a monovalent olefinically unsaturated hydrocarbon or hydrocarbon- oxy radical, each Y represents a hydrolysable organic radical and R' represents an R radical, a methyl radical or a Y radical) in the presence of a compound capable of generating free radical sites in the polyolefin, the said reaction being carried out at a temperature above 140°C and the said free radical generating compound having a half-life at the reaction temperature of less than 6 minutes; and subsequently exposing the product of to moisture in the presence of a silanol condensation catalyst. We have now discovered that the process described and claimed in said application is also applicable to the preparation of certain cross-linkable and cross-linked copolymers of ethylene and alkyl acrylates. Accordingly this invention provides a process for the production of a cross-linked copolymer of ethylene and one or more alkyl acrylates which comprises reacting a

copolymer of ethylene and one or more alkyl acrylates having from 4 to 7 inclusive carbon atoms in the molecule, said copolymer containing up to 40 per cent by weight of units

2 463,5 0 0

60.862/ I

derived from one or more alkyl acrylates, with 0.1 to

by weight of the copolymer of a silane of the formula RP'SiY 2 wherein R represents a monovalent olefinically unsaturated radical attached to silicon through a silicon to carbon bond

and composed of carbon, hydrogen and, optionally, oxygen, each Y represents a hydrolysable organic radical and R' represents a monovalent hydrocarbon radical free of aliphatic unsaturation or a Y radical in the presence of 0.005 to 1.5% by weight of the copoly-

mer of a free radical generating compound, said reaction being

carried out at a temperature above 140°C and the said free readical generating compound having a half-life at the reaction temperature of less than 6 minutes and subsequently exposing the product of to water in the presence of a silanol condensation catalyst. The invention also includes cross-linked poly(ethylene- alkyl acrylate) whenever produced by the said process.

The process of this invention may be employed to effect the cross-linking of ethylene-alkyl acrylate copolymers containing up to 40 per cent by weight of copolymeric units derived from one or more alkyl acrylates, said alkyl acrylates having from 4 to 7

inclusive carbon atoms in the molecule. The preferred copolymers are those containing from 5 to 25 per cent by weight of units

dervied from the alkyl acrylate. It is also preferred that the alkyl acrylate units are derived from ethyl acrylate.

If desired there may also be included with the ethylene-alkyl

acrylate copolymer a proportion of an ethylene polymer free of

units derived from an alkyl acrylate. The proportion of the latter employed may vary widely depending on the diluent

effect required but will normally not exceed that of the copolymer.

463..500

60,862 /73 In the general formula of the silanes employed In the process of this. invcntion,, R -represe~nts a. monovalent, olef inically unsaturated radical composed of carbon, hydrogen and,, optionally, oxygen. Examples of such radicals are vinyl.' allyl, butenyl, cyclohexenyl, cyclopentadiecnyl, cyclohexadienyl' CH C( CH3 )COO(C1 3

CH2 =C(CH 3 )COOCH 2 CH 2 O(CH2 3 at d CH =(H)COodH CH OCH 2 O(CH) 3 the vinyl radical being preferred. The group Y may represent any hydrolysable organic radical, for example an alkoxy radical, such as the~ methoxy, ethoxy and butoxy radicals, an acyloxy radical, for example the formyloxy, acetoxy or propionoxy radicals, an oximo radical e.g. -O17-C(CH3 2 7. -ON'=C(CH 3 )H 5 and -M=~652or a. substituted amino radical e.g. alkylamino and arylamino radicals, example.s of which are -NHCH 3 NHC 2 H5 and -NH(CCH 5 The Y substituents in any given silane molecule may be the same or different. The group RI may represent a monovalent hydrocarbon radical free of aliphatic unsaturation e.g. methyl, ethyl, propyl, tetradecyl, octadecyl, phenyl, benzyl or tolyl. It may also represent a Y group. Preferably the silane will have the formula RSiY 3wherein R is vinyl, the most preferred silanes being vinyltriethoxysilane, vinyl trimethoxysilane and combinations of these. Silanes having only two hydrolysable groups e.g. -vinylmethyldiethoxysilane and vinyiphenyldi- .methoxysilane, are however, also operative. The proportion of silane employed will depend in part upon the reaction conditions and in part on the degree of modification desired in the ethylene-alkyl aerylate

463,500

60.862 7

eopolymer. The actual proportion may vary widely, for example from 0.1 to 20 per cent by weight based on the

weight of copolymer or of both copolymer and polyethylene when the latter is present.In general., however, we based prefer to employ from 0.5 to 5 per cent by weight on the weight of the copolymer. As the free radical generating compound there may be radical employed any compound which is capable of producing under the sites in the ethylene-alkyl acrylate copolymer the reaction conditions and which has a half-life at reaction temperature of less than 6 minutes, and preferably less than 1 minute. The best known and preferred free are radical generating compounds for use in this invention the organic peroxides and peresters, for example benzoyl peroxide, dichlorobenzoyl peroxide, dicumyl peroxide, di-tertiary butylperoxidq, 2,5-dimethyl-2,5 di(peroxy- benzoate)hexyne-3; 1,3-bis(t-butylperoxyisopropyl)benzene, lauroyl peroxide, tertiar buty pracetat -dmt

2,5-t--butylperoxy)hexyne-3; 2 5 and azo compounds peroxy)hexane and tertiarybutyl perbenzoate, for example azobis-isobutyronitrile and dimethylazodi- isobutyrate, dicumyl peroxide being the most preferred. chosen The particular free radical generating compound at which in any instance will depend upon the temperature with the reaction of the ethylene-alkyl acrylate copolymer silane is to be performed. Thus if the reaction is to be benzoyl peroxide or performed at, say, from 160 to 180C, at dicuayl peroxide, which have appropriate half-lives these temperatures, can be used. should be used Sufficient of the free radical generator

4.6 3.5 0 0

60,862/73

to obtain the desired degree of modification of the be so copolymer. Houever, the proportion should not or free- high as to cause degradation of the copolymer the radical cross-linking of the copolymer to become

.prdoninan 8anl"sm. The proportion required will free radical depend to some extent on the nature of the be employed genart itself. Some peroxide compounds may in relatively high proportions, e.g. up to 1.5% by weight

based on the weight of copolymer without causing to occur. significant conventional peroxide cross-linking is However, when the free radical generating compound effective in producing such conventional cross-linking by we prefer to employ not more than about 0.75 per cent based on the weight of the free radical generating compound per cent of the weight of the copolymer. As little as 0.005 to free radical generator may be used and will give rise some cross-linking capability in the.ethylene-alkyl For acrylate copolymer in the process of this invention. to most applications, however, we prefer to use from 0.2 per cent by weight based on the weight of the copolymer.. In accordance with Step of the process of this

invention reaction between the ethylene-alkyl acrylate copolymer and the silane is carried out at a temperature above 140°C. .The reaction may be carried forward employing reaction any suitable apparatus. Preferably, however, the is carried forward under conditions in which the copolym.er is subjected to mechanical working. The reaction is therefore preferably carried out in, for example, an extruder, an internal Banbury mixer or a roll mill provided it permits the copolymer to reach the desired

463.500

608 6 2/

temperature. Banbury is a Registered Trade Park.). The most preferrred form of apparatus is that of the type which comprises a closed extruder adapted to effect a kneading or compounding action on its contents. One such type of iSppam1u.s is

that known as the Ko-Kneader which is adapted to accomplish

mechanical working and compounding of plastics at elevated

temperatures. The ethylene-alkyl acrylate copolymer and silane

reactants may be brought together by any convenient means. For example the liquid silane may be introduced into the apparatus in which the reaction is to take place dispersed on the surface of the copolymer or it may be metered directly into the apparatus. The free radical generating compound may also be introduced by way 6f the surface of the copolymer or may be added, when solubility considerations permit, as a solution in the silane.

Reaction between the silane ad the ethylene-alkyl acrylate copolymer may be carried forward at any temperature between 140 0 C and the degradation temperature of the copolymer. The actual reaction temperature employed will normally be

determined by considerations of the type of apparatus in which

the reaction is performed and on the power input for the

apparatus. It will also depend on the temperature at which

degredation of the copolymer occurs. We prefer to perform the 0 reaction at temperatures in the range from about 140 to 220 C for periods of up to 10 minutes. Although the minor amounts of organic solvents may

be employed if desired to facilitate the addition of the

free radical generator or the silane the reaction is

463.50o

60,862

usually best carried out in bulk since this avoids subsequent

tedious solvent removal procedures.

Cross-linking of the silane-modified ethylene-alkyl acrylate copolymer according to Step of the process of this invention is effected by exposing the product of Step usually after shaping by extrusion or other processes,':to water in the presence of a silanol condensation catalyst. The m6isture present in the atmosphere is usually sufficient for cross-linking to occur but the rate of cross-linking may be hastened if desired by the use of an artifically moistened atmosphere, immersion in liquid water, optionally employing elevated temperatu-

res, or by exposure to steam. Preferably crosslinking is 0 effected at a temperature between 50°C and about 80 C. Provided the reaction product of the ethylene-alkyl

acrylate copolymer and the silane is kept free of moisture

and/or of material which acts as a silanol condensation catalyst

it may be stored if desired. As it is often difficult to exclude

all traces of moisture from the copolymer the preparation of a prod-

uct which remains substantially unchanged when stored is usally on]

possible when the hydrolysable moieties in the silane are

those, such as alkoxy radicals, which give rise to noncatalytic

by-products. When the silane contains, for example, silicon- bonded acetoxy radicals or even trace quantities of silicon- bonded chlorine atoms the acetic acid or hydrochloric acid by-

products can cause some cross-linking to occur at normal

ambient temperatures if moisture has not beenexcluded from the

composition.

If a storage stable product is not desired the use of

463.500

60,862 /73 generates a silanol Ssilane or other additive which hydrolysis may be :ondensation catalyst In situ during In general, however, we )ermissable, or even preferred. condensation catalyst refer to incorporate the silanol Lnto the product of Step only when it is desired to

product, for example, just initiate cross-linking of the article. Thus, for prior to shaping into the finished example Step the addition of the silanol condensation product may catalyst and shaping of the. cross-linkable by performing all of take place continuously, for example extruder. the operations in a single compounding copolymer composition can Alternatively the cross-linkable separate components, one be provided in the form of two which has been modified component comprising the copolymer the according to Step of the process of this invention, obtained by other component comprising a composition catalyst with ethylene- mixing the silanol condensation desired polyethylene alkyl acrylate copolymer, or if by reaction with the itself which has not been modified then be blended in the silane. The two components may a composition which is desired proportions to provide water. When a copolymer cross-linkable in the presence of capability is having the maximum degree of cross-linking or copolymer preferably desired the unmodified polymer 10 per cent by weight of the comprises not mcre than about acrylate copolymer in the total quantity of ethylene-alkyl a cross-linkable mixed composition. .ihen preparing therefore we prefer to employ copolymer by.this technique weight of the unmodified from about 1 to 10 per cent by on the total copolymer copolymer catalyst component based

n 463,50

60,862/73 content of the compositic. Preferably also the polyethylene or copolymer used in the catalyst composition has flow characteristics which are the same as, or approximate to, that of the silane-modified copolymer with which it is to be mixed. Any of the wide variety of materials which function as silanol condensation catelysts may be employed in the process of this invention. Such materials include, for example, metal carboxylates such as dibutyltin dilaurate, stannous acetate, stannous octoate, lead naphthenate, zinc octoate, iron-2-ethylhexoate and cobalt naphtehnate, organic metal compounds such as the titanium esters and chelates, for example tetrabutyl titanate, tetranonyl titanate and bis(acetylacetonyl)di-isopropyl titanate, organic bases such as ethylamine, hexylamine, dibutylamine and piperidine and acids such as the mineral acids and fatty acids. The preferred catalysts are the organic tin compounds for example, dibutyltin dilaurate, dibutyltin diacetate and dibutyltin dioctoate. Having regard to the considerations discussed hereinabove the silanol condensation catalyst may be incorporated prior to, during, or following the reaction of the ethylene-alkyl acrylate copolymer and the silane. The catalyst may be added as such or generated in situ either by hydrolysis of the silane, or, for example, by the thermal decomposition of a material which liberates an amine or other silanol condensation catalyst as a decomposition product. Another method of employing the catalyst which can be used is described in our co-pending Application No.

463.500

60,862/73 147985/72 and comprises contacting the product of

Step with an aqueous dispersion or solution of a tin

carboxylate..

In addition to the presence of the silanol

condensation catalyst,,or.precursor .thereof,.the cqpolymer

may contain if desired other ingredients, for example black. pigments, anti oxidants and fillers, e.g. carbon Cross-linkable ethylene-alkyl acrylate copolymers prepared

according to this invention may also be blended with er sce polyethylene or chlorinated polyethylene either

or which has been rendered cross-linkable by a similar

process.

Cross-linked ethylene-alkyl acrylate copolymer

produced according to this invention possesses improved

compression set resistance and air oven ageing properties.

The process of the invention may therefore advantageously and be employed in the provision of electrical insulation

components or other extruded articles such as containers

and conduits for hot liquids. It is also applicable to

the preparation of cross-linked adhesives. The following example, in which the parts are expressed

by weight, illustrates the invention.

Example. in this The ethylene-alkyl acrylate copolymer employed 6169 (Union example was that available commercially as DPD weight of Carbide) containing approximately 18 per cent by 0 6.3g./1 min. ethyl acrylate and having a'melt flow index of

100 parts of this copolymer in chip form were tumbled with in 2.0 parts a solution of 0.308 parts of dicumyl peroxide was of vinyl trimethoxysilane until all of the solution

11 463.5 00

862/73

dispersed over the copolymer surface. This material was then passed through a Buss PR 46 Ko-Kneader at a rate of

approximately 110 gmas./min., both the kneading and granulating screws of the apparatus being operated at a temperature of 200 0 C.

The polymer (melt flow index 1.5 g./10 min.) was recovered in the form of pellets.

parts of the copolymer graft were compounded with parts of unlodified polyethylene (shell"Carlona 25020 Carlona is a Registered Trade Mark) containing 0.25 parts dibutyl tin dilaurate using a Brabender Plastograph at 130 0 C.

(Brabander Plastograph is a Registered Trade Mark). The compounded pieces of polymer after removal from the Brabender Plastograph were pressed at 180 0 C into uniform sheets

having a thickness of 3 mm. Cross-linking of the copolymer sheets was brought about 0 by Immersing them in water at 80 C for 20 hours.

The degree of cross-linking obtained in each case was determined

by measuring the amount of insoluble material in the treated sheet. This measurement was carried out by extraction of the material in boiling xylene for 20 hours.

The result obtained are shown below together with

a comparative value for unmodified copolymer.

Material Gel Content

Silane Modified Copolymer 76 Unmodified copolymer 0

4635 00

Claims

60,862/73

THE CLAIMS DEFINING THEIS INVENTION ARE AS FOLLOWS:

1. A process for the production of a cross-linked copolymer

of ethylehe and one or more alkyl acrylates which comprises

reacting a copolymer of ethylene and one or more alkyl

acrylates having from 4 to 7 inclusive carbon atoms in the molecule, said copolymer containing up to 40 per cent with by weight of units derived from one or more alkyl acryltes 0.1 to 20% by weight of the copolymer of a silane of the formula

RR' SiY 2 wherein R represents a monovalent olefinically unsaturated

radical attached to silicon through a silicaon to carbon

bond and composed of carbon, hydrogen and, optionally, oxygen,

each Y represents a hydrolysable organic radical and R' represents or a monovalent hydrocarbon radical free of aliphatic unsaturation a Y radical in the presence of 0.005 to 1.5% by weight of the being copolymer of a free radical generating compound, said reaction 0 said free radical carried out at a temperature above 140 C and the

generating compound having a half-life at the reaction temperature of less than 6 minutes and subsequently exposing the product

of tO water in the presence of a silanol condensation catalyst.

2. A process as claimed in claim 1 wherein the ethylene-

alkyl acylate copolymer is an ethylene-ethyl acrylate copolymer. 3. A process as claimed in claim 1 or claim 2 wherein

the ethylene-alkyl acrylate copolymer contains from 5 to to

per cent by weight of units derived from the alkyl acrylate.

4. A process as claimed in any of the preceding claims the wherein the silane has the formula RSiY 3 in which R represents

vinyl radical and Y is as defined in claim 1.

463,500

60.862/73

A process as claimed in claim 5 whereinthe silane is vinyltrimethoxysilane and/or vinyltriethoxysilane. 6. A process as claimed in any one of the preceding claims wherein the silane is employed is employed in a proportion of from 0.5 to 5 per cent by weight based on the weight of 7. A process as claimed in any one of the preceding claims wherein the silanol condensation catalyst is an organic tin compound. 8. A process as claimed in any one of the preceding claims wherein the free radical generator is employed in a proportion of from 0.005 to 0.75 parts based on the weight of

copolymer.

9. A process for the production of a cross-linked copolymer of ethylene and one or more alkyl acrylates substantially as described

with reference to the Example.

A cross-linked copolymer of ethylene and one more alkyl

acrylates whenever obtained by the process claimed in any one of

the preceding claims.

DATED THIS 28th DAY OF SEPTEMBER 1973.

DOW CORNING LIMITED.

463.500