WO1991009075A1

WO1991009075A1 - Cross-linkable polymer composition containing an acid anhydride as a silanol condensation catalyst

Info

Publication number: WO1991009075A1
Application number: PCT/SE1990/000733
Authority: WO
Inventors: Bernt-Åke SULTAN; Thomas Hjertberg; Magnus Palmlöf
Original assignee: Neste Oy
Priority date: 1989-12-13
Filing date: 1990-11-14
Publication date: 1991-06-27
Also published as: SE465165B; SE8904196L; SE8904196D0

Abstract

A polymer composition containing a cross-linkable polymer and at least one silanol condensation catalyst is characterised in that the silanol condensation catalyst is an acid anhydride, preferably a carboxylic acid anhydride, for example selected amongst aliphatic carboxylic acid anhydrides, aromatic carboxylic acid anhydrides and polymer carboxylic acid anhydrides. Stearic anhydride and benzoic anhydride are examples thereof. The acid anhydride catalyst may also be used in combination with conventional silanol condensation catalysts, such as dibutyl tin dilaurate.

Description

Cross-linkable polym^pr composition containing an acid anhydride as a silanol condensation catalyst.

The present invention relates to a cross-linkable polymer composition containing a cross-linkable polymer with hydrolysable silane groups and at least one silanol condensation catalyst.

It is known to cross-link different polymers by means of additives. Crosslinking improves such properties of the polymer as its mechanical strength and heat resistance. Polymers normally considered to be thermoplastics, and not "cross-linkable, can also be cross-linked by introducing cross-linkable groups in the polymer. An example thereof is the crosslinking of polyolefins, such as polyethylene. A silane compound can be introduced as a cross-linkable group, e.g. by grafting the silane compound onto the pre¬ pared polyolefin, or by copolymerisation of the olefin and the silane compound. This technique is previously known, and further details may be obtained from US Patent Speci¬ fications 4,413,066; 4,297,310; 4,351,876,- 4,397,981; ^* 4,446,283; and 4,456,704, all of which are incorporated herein by reference.

The crosslinking of polymers with hydrolysable silane groups is carried out by so-called moisture curing. In a first step, the silane group is hydrolysed under the ^'influence of water, resulting in the splitting-off of alcohol and the formation of silanol groups. In a second step, the silanol groups are cross-linked by a condensa¬ tion reaction splitting off water. In both steps, a so- called silanol condensation catalyst is used as catalyst. Prior art silanol condensation catalysts include carboxylates of metals, such as tin, zinc, iron, lead and cobalt; organic bases; inorganic acids; and organic acids. Mention should here especially be made of dibutyl tin dilaurate, dibutyl tin diacetate, dioctyl tin dilaurate, stannous acetate, stannous caprylate, lead naphthenate, zinc caprylate, cobalt naphthenate, ethyl amines, dibutyl amine, hexylamines, pyridine, inorganic acids, such as sulphuric acid and hydrochloric acid, as well as organic acids, such as toluene sulphonic acid, acetic acid, stearic acid and maleic acid. Especially the tin carboxy- lates are much used as catalysts. GB 2,028,831 and EP 0,193,317 may be mentioned as examples of the prior art relating to the crosslinking of polymers containing hydrolysable silane groups by means of the above-mentioned silanol condensation catalysts. EP 0,207,-627 also discloses the use of a special silanol condensation catalyst in the form of a tin-containing polymer.

JP 60013804 further discloses a curable, heat-resis¬ tant adhesive composition comprising a modified polyolefin and a silanol condensation catalyst. The modified polyole- fin is formed from polyolefin, an ethylenically unsatu- rated silane compound, and an unsaturated carboxylic acid or a derivative thereof which for instance may include ( eth)acrylic acid, maleic anhydride, itaconic acid, alkyl(meth)aerylate, maleicimide or (meth)acrylamide. It should be pointed out that the maleic anhydride, if used, does not constitute a silanol condensation catalyst, but forms part of the cross-linkable polymer proper.

Although the above silanol condensation catalysts, and in particular the tin carboxylates, are frequently used in the crosslinking of polymer compositions contain¬ ing silanol groups, they are disadvantageous in some respects. Thus, efforts are being made to find silanol condensation catalysts reducing or obviating these disad¬ vantages. For instance, the prior art silanol condensation catalysts often result in unwanted and premature cross- linking, so-called scorching or precuring, of the polymer composition. Such precuring arises, for example, on extru¬ sion of the polymer composition to which the silanol ^* con- densation catalyst has been added, and makes it impossible to maintain a steady production rate and also impairs the quality of the resulting product. Crosslinking or precuring which sets in already in the extruder (or similar equipment) causes gel formation and the adhesion of polymer gel. Thus, the equipment must be cleaned from adhesive polymer gel and, on every such occasion, be shut down, with ensuing production losses. Another disadvantage is that the gel clots formed which do not get stuck in and clog the manufacturing equipment are fed out and appear as disfiguring and unwanted lumps in the product. In thin layers, such as films and foils, such lumps are totally unacceptable and mostly make the product unusable.

The unwanted precuring may be counteracted by intro¬ ducing in the polymer composition substances counteracting precuring, i.e. so-called precuring retarders, as describ- ed in the above EP 0,193,317.

Although it is possible to thus reduce the unwanted precuring, the addition of a precuring retarder involves an additional working operation and the introduction of a further component in the composition, which increases the work contribution required, as well as the costs.

Another disadvantage of prior art silanol condensa¬ tion catalysts, such as the tin carboxylates most fre¬ quently used to date, is that they are toxic. Especially when producing cross-linked polymer material for use in connection with food or pharmaceuticals, it would be pre¬ ferable to replace these toxic catalysts by other, non- toxic or at least less toxic catalysts.

The present invention concerns a new type of silanol condensation catalysts reducing or obviating the incon- veniences associated with the use of prior art catalysts for crosslinking polymer compositions containing polymers with hydrolysable silane groups. More precisely, the inventive silanol condensation catalyst is an acid anhy¬ dride, such as a carboxylic acid anhydride. Thus, the invention provides a cross-linkable polymer composition containing a cross-linkable polymer with hydrolysable silane groups and at least one silanol con- densation catalyst, characterised in that the silanol con¬ densation catalyst is an acid anhydride.

Further characteristics of the invention are apparent from the claims and the text below. According to the invention, it is preferred that the acid anhydride used is a carboxylic acid anhydride.

According to the invention, the at present most pre¬ ferred carboxylic acid anhydrides are the aliphatic car¬ boxylic acid anhydrides, especially the anhydrides of ali- phatic carboxylic acids having 2-20 carbon atoms, such as acetic acid, propionic acid, butyric acid, stearic acid, lauric acid, and caprylic acid. Also mixtures thereof, such as fatty-acid anhydride mixtures, may be used.

The invention further comprises aromatic carboxylic acid anhydrides, such as benzoic anhydride, ^"o-phthalic anhydride and trimellitic anhydride; dicarboxylic anhy¬ drides, such as succinic anhydride, maleic anhydride, tetrahydrophthalic anhydride; and polymer acid anhydrides, such as acid anhydrides derived from copolymers of ethy- lene/acrylic acid, ethylene/methacrylic acid, ethylene/ maleic acid. It will be appreciated that the latter are but a few examples of polymer acid anhydrides, and that other polymer acid anhydrides may be used which have been obtained from two or more different monomers, of which at least one is an acid anhydride or contains carboxyl groups that may be transformed into acid anhydride groups. Apart from copolymerisation, the monomers forming the acid anhy¬ dride may be introduced by graft polymerisation.

According t^'o the invention, the amount of silanol condensation catalyst present in the cross-linkable poly¬ mer composition generally is in the order of about 0.001-10% by weight, preferably about 0.01-5% by weight, especially about 0.01-1.5% by weight, as based on the amount of silanol-group containing polymers in the compo- sition. It will be appreciated that the effective amount of acid anhydride catalyst depends on the molecular weight of the acid anhydride, more precisely the number of acid anhydride groups per mole of acid anhydride. Thus, a smaller amount is required of an acid anhydride having many acid anhydride groups and a low molecular weight, than of an acid anhydride having but few acid anhydride groups and a high molecular weight.

The inventive acid anhydride catalyst is preferably added to the cross-linkable polymer in the form of a master batch, i.e. mixed with a polymer, such as polyethy¬ lene. The master batch contains a minor amount of the acid anhydride catalyst, generally about 1-10% by weight, pre¬ ferably about 3-5% by weight.

The polymer acid anhydride catalysts according to the invention are especially advantageous, since the catalysts may be added directly to the cross-linkable polymer, there. being no need of first producing a master batch. Unlike conventional low-molecular silanol condensation catalysts, the polymer acid anhydride catalyst according to the invention has no or only a slight tendency to migrate. This is further advantageous when the polymer acid anhy- dride catalyst and the cross-linkable polymer are mixed in the form of pellets. Since both the cross-linkable polymer and the polymer acid anhydride catalyst are in pellet form, there is no intimate contact on a 'molecular level' between them. The polymer acid anhydride catalyst further- more has no or only a slight tendency to migrate,, and there is thus no risk of such intimate contact arising with time. It is therefore possible to mix pellets of the cross-linkable polymer and the polymer acid anhydride catalyst as early as at the factory without risking unwanted, premature crosslinking. Thus, it is not neces¬ sary to mix them with the polymer in connection with the extrusion, as is the case with conventional low-molecular silanol condensation catalysts because of the risk of catalyst migration and premature crosslinking of the poly- mer composition. It will immediately be appreciated that this substantially simplifies the operation, and thus is advantageous to the user. In this context, it ^"should, however, be pointed out that this implies that the mole¬ cule of the polymer acid anhydride catalyst does not con¬ tain any hydrolysable silane groups. If this were the case, the conditions for intimate contact on 'molecular level' between the acid anhydride catalyst and the silane groups would be present, involving the risk of crosslink¬ ing of the silane groups. Besides, it is a general con¬ dition that the molecule of the acid anhydride catalyst according to the invention be free from silane groups, to avoid the risk of unwanted crosslinking of such silane groups.

The inventive acid anhydride catalyst may be used in the cross-linkable polymer composition alone or combined with other silanol condensation catalysts, which may be other acid anhydride catalysts or conventional silanol condensation catalysts, such as carboxylic acid salts of the metals tin, zinc, iron, lead and cobalt; organic bases; inorganic acids; and organic acids. This combina¬ tion of an acid anhydride catalyst and a conventional silanol condensation catalyst results in an unexpected synergism, as will be described in more detail below.

When the silanol condensation catalyst according to the invention is added to the cross-linkable polymer com¬ position in connection with the extrusion of the composi- tion, the silanol condensation catalyst should have a boiling point exceeding the treatment temperature during extrusion, thereby to prevent the catalyst from escaping. Thus, the boiling point of the acid anhydride catalyst should be at least about 120°C, preferably at least about 220°C.

The cross-linkable polymer composition according to the invention chiefly corresponds to prior art cross-link¬ able polymer compositions containing hydrolysable silane groups, but differs therefrom by containing the acid anhy- dride catalyst described above. Thus, the invention generally concerns cross-linkable polymers containing hydrolysable silane groups, and more precisely it relates to olefin copolymers or graft poly¬ mers which contain hydrolysable silane groups and which are cross-linked under the influence of water and at least one silanol condensation catalyst. Preferably, the cross- linkable polymer is an ethylene homopolymer or copolymer containing cross-linkable silane groups introduced either by copolymerisation or graft polymerisation. Preferably, the silane-containing polymer has been obtained by copolymerisation of an olefin, suitably ethy¬ lene, and an unsaturated silane compound represented by the formula

RSiR'_nY₃__n , ^• . (I) wherein

R is an ethylenically unsaturated hydrocarbyl, hydrocar- byloxy or (meth)acryloxy hydrocarbyl group, R' is an aliphatic saturated hydrocarbyl group, Y which may be same or different, is a hydrolysable orga- nic group, and n is 0, 1 or 2.

If there are more than one Y group, these do not have to be identical.

Special examples of the unsaturated silane compound ^' are those wherein R is vinyl, allyl, isopropenyl, butenyl, cyclohexenyl or gamma-(meth)acryloxy propyl; Y is methoxy, ethoxy, formyloxy, acetoxy, propionyloxy or an alkyl- or arylamino group; and R' is a methyl, ethyl, propyl, decyl or phenyl group. A preferred unsaturated silane compound is repre¬ sented by the formula

CH₂=CHSi(OA)₃ (II) wherein A is a hydrocarbyl group having 1-8 carbon atoms, - preferably 1-4 carbon atoms. The most preferred compounds are vinyl trimethoxy- silane, vinyl bismethoxyethoxysilane, vinyl triethoxy- silane, gamma-(meth)acryloxypropyltrimethoxysilane, gamma- (meth)acryloxypropyltriethoxysilane, and vinyl triacetoxy- silane.

The copolymerisation of the olefin (ethylene) and the unsaturated silane compound may be carried out under any suitable conditions resulting in the copolymerisation of the two monomers.

Moreover, the copolymerisation may be implemented in the presence of one or more other comonomers which can be copolymerised with the two monomers. Such comonomers include (a) vinyl carboxylate esters, sucri as vinyl ace¬ tate and vinyl pivalate, (b) alpha-olefins, such as pro- pene, 1-butene, 1-hexene, 1-octene and 4-methyl-l-pentene, (c) (meth)aer lates, such as methyl(meth)aerylate, ethyl- ( eth)acrylate and butyl(meth)acrylate, (d) olefinically unsaturated carboxylic acids, such as (meth)acrylic acid, maleic acid and fumaric acid, (e) (meth)acrylic acid deri¬ vatives, such as (meth)aerylonitrile and (meth)acrylic amide, (f) vinyl ethers, such as vinyl methyl ether and vinyl phenyl ether, and (g) aromatic vinyl compounds, such as styrene and alpha-methyl styrene. Amongst these comono¬ mers, vinyl esters of monocarboxylic acids having 1-4 car¬ bon atoms, such as vinyl acetate, and (meth)acrylate of alcohols having 1-4 carbon atoms, such as methyl(meth)- acrylate, are preferred. Especially preferred comonomers are butyl acrylate, ethyl acrylate and methyl acrylate.

Two or more such olefinically unsaturated compounds may be used in combination. The term ' (meth)acrylic acid' is intended to embrace both acrylic acid and methacrylic acid. The comonomer content of the copolymer may amount to 70% by weight of the copolymer, preferably about 0.5-35% by weight, most preferably about 1-25% by weight.

If using a graft polymer, this may have been produced by any of the two methods described in US 3,646,155 and US 4,117,195, respectively. The silane-containing polymer according to the inven¬ tion suitably contains 0.001-15% by weight of the silane compound, preferably 0.01-5% by weight, most preferably 0.1-3% by weight.

As is usually the case for polymer compositions, the cross-linkable polymer may contain various additives, such as miscible thermoplastics, stabilisers, lubricants, fil¬ lers, colouring agents and foaming agents.

As to the miscible thermoplastics added, mention may be made of miscible polyolefins, such as polyethylene of low density, medium density and high density, polypro- pene, chlorinated polyethylene, as well as various copo- lymers including ethylene and one or more other comono¬ mers, e.g. vinyl acetate, methyl acrylate, propene, butene, hexene and the like. One may use either a single polyolefin or a mixture of several polyolefins. The poly- olefin content of the composition may be up to 70% by weight, as based on the total amount of this polyolefin and the silane-containing polymer.

As to fillers, mention may be made of inorganic fil¬ lers, such as silicates, e.g. kaolin, talc, montmorillo- nite, zeolite, mica, silica, calcium silicate, asbestos, powdered glass, glass fibre, calcium carbonate, gypsum, magnesium carbonate, magnesium hydroxide, carbon black and titanium oxide. The content of the inorganic filler may be up to 60% by weight, as based on the sum of the weights of the filler and the silane-containing polymer.

Finally, it should be observed that the cross-link¬ able polymer used in the invention is previously known and that the novel and distinctive features of the invention thus do not reside in this polymer, but in the special silanol condensation catalyst added to the polymer to form the inventive polymer composition.

The following nonrestrictive Examples are incorpo¬ rated herein to further illustrate the invention. EXAMPLE 1

The starting material was a cross-linkable polymer having hydrolysable silane groups and being a copolymer of ethylene and vinyl trimethoxysilane. The vinyl trimethoxy- silane content was 1.9% by weight. A cross-linkable compo¬ sition was produced by adding to the polymer a silanol condensation catalyst made up benzoic anhydride. This anhydride was added in the form of a master batch of low- density polyethylene containing 5% by weight of benzoic acid. The master batch was added in such amounts that the content of benzoic acid, as based on the entire composi¬ tion, was about 0.05% by weight. Then, the composition was cross-linked at a temperature of about 90°C in the pre¬ sence of water, and the degree of crosslinking of the com- position was measured on different occasions by extraction with decalin according to the method UNI ^'459 which corre¬ sponds to ASTM D 2765, except that the extraction with decalin after 6 h is continued for yet another hour in pure boiling decalin. The results obtained are apparent from Table 1, and the values given are mean values of two determinations.

TABLE 1

One proceeded as in Example 1, the only difference being that the content of benzoic anhydride was about 0.5% by weight, as based on the entire composition. The results obtained are apparent from Table 2.

TABLE 2

EXAMPLE 3

One proceeded as in Example 1, the only difference being that stearic anhydride was used as silanol conden¬ sation catalyst instead of benzoic anhydride. The content of stearic anhydride was about 0.05% by weight, as based on the entire composition. The results obtained are appa¬ rent from Table 3.

TABLE 3

0

One proceeded as in Example 3, the only difference

15 being that the content of stearic anhydride was about 0.5% by weight, as based on the entire composition. The results obtained are apparent from Table 4.

TABLE 4

20

-,-- One proceeded as in Example 1, but a polymer acid anhydride was used as silanol condensation catalyst instead of benzoic anhydride. The polymer acid anhydride was a copolymer of propene with about 11% by weight of maleic anhydride and about 10% by weight of ethyl acry-

₃₀ late. This polymer acid anhydride is sold by the firm Norsolor under the trade name of EMSA. The polar acid anhydride was added to the cross-linkable ethylene/vxnyl trimethoxysilane copolymer, such that the content of polymer acid anhydride amounted to about 5% by weight, as

₃₅ based on the total composition. Then, the composition was cross-linked, and the degree of crosslinking was deter¬ mined in the manner stated in Example 1. The results obtained are apparent from Table 5.

This Example illustrates the synergistic effect obtained when using the inventive acid anhydride catalyst in combination with a conventional silanol condensation catalyst.

One proceeded as in Example 1, and three parallel tests with different silanol condensation catalysts were carried out. The first test used a conventional silanol condensation catalyst consisting of dibutyl tin dilaurate^' (DBTDL). The DBTDL content was about 0.05% by weight, as based on the entire composition. The second test used an acid anhydride catalyst according to the invention, namely stearic anhydride (SSA) in a content of about 0.05% by weight, as based on the entire composition (cf. Example 3 above). The third test used a combination of stearic anhy¬ dride (SSA) and DBTDL as silanol condensation catalyst. Thus, 0..05% by weight, as based on the entire composition, was added of each catalyst. In all the tests, the cata¬ lysts were present as a master batch with polyethylene, the SSA master batch having a catalyst content of 5% by weight, and the DBTDL master batch having a catalyst con¬ tent of 0.5% by weight^'. By following the method described in Example 1, the results given in Table 6 were obtained. 20 .40

73 . 4

0 ( 5 h ) 26 . 9 61 . 5

73 . 3

These results clearly show that simultaneous use "of stearic anhydride and DBTDL as silanol condensation cata- lyst resulted in a more rapid and stronger crosslinking than separate use of the catalysts. This effect is espe¬ cially pronounced at the beginning of the crosslinking, and is of considerable practical importance since it entails that dimensional stability of the extruded product is^* obtained more rapidly when crosslinking in connection with e.g. extrusion of a polymer composition.

EXAMPLE 7

Also this Example illustrates the synergistic effect obtained when using the acid anhydride catalyst according to the invention in combination with a conventional silanol condensation catalyst.

One proceeded as in Example 6, the only difference being that the content of stearic anhydride catalyst in the composition was increased from 0.05% by weight to 0.5% by weight. The results obtained are shown in Table 7. TABLE 7

Time of crosslinking (h) 0 20 40

Degree of crosslinking (%)

Test 1

(0.05% DBTDL) 0 54.9 61.8 69.1 73.4

Test 2

(0.5% SSA) 0 22.1/5 h 63.9 69.

Test 3

(0.05% DBTD1+ 0.5% SSA) 0 51.7 58.4 63.2 69.1

Test 1 adjusted in view 0 48.7 54.9 61.4 65.2 of the diluting effect in Test 3

The results in Table 7 are somewhat difficult to interpret owing to the diluting effect in Test 3, which renders misleading a direct comparison with Tests 1 and 2. In Tests 1 and 2, about 10% by weight of the polymer com¬ position consisted of a master batch (including the cata¬ lyst), while in Test 3, as much as 20% by weight of the polymer composition consisted of a master batch (10% by weight of DBTDL master batch + 10% by weight of SSA master batch). To correct this, the values in Test 1 (0.05% by weight of DBTDL) were multiplied by 8/9, and the values thus obtained, which are comparable with the values in Test 3, are given at. the bottom of Table 7 as 'Test 1 adjusted in view of the diluting effect in Test 3'. A com¬ parison of these adjusted values with the values of Test 3 clearly shows the synergistic effect obtained when using the acid anhydride catalyst according to the invention together with a convention silanol condensation catalyst. It is also obvious that this effect is especially pro¬ nounced at the beginning of the crosslinking.

Claims

1. A cross-linkable polymer composition containing a cross-linkable polymer with hydrolysable silane groups and at least one silanol condensation catalyst, c h a r a c ¬ t e r i s e d in that the silanol condensation catalyst is an acid anhydride.

2. The cross-linkable polymer composition of claim 1, c h a r a c t e r i s e d in that the acid anhydride is a carboxylic acid anhydride.

3. The cross-linkable polymer composition of claim 2, c h a r a c t e r i s e d in that the acid anhydride is an aliphatic carboxylic acid anhydride.

4. The cross-linkable polymer composition of claim 3, c h a r a c t e r i s e d in that the acid anhydride is the anhydride of a saturated fatty acid having 8-18 carbon atoms.

5. The cross-linkable polymer composition of claim 2, c h a r a c t e r i s e d in that the acid anhydride is an aromatic carboxylic acid anhydride.

6. The cross-linkable polymer composition of claim 5, c h a r a c t e r i s e d in that the acid anhydride is benzoic anhydride.

7. The cross-linkable polymer composition of claim 2, c h a r a c t e r i s e d in that the acid anhydride is a polymer carboxylic acid anhydride.

8. The cross-linkable polymer composition of any one of the preceding claims, c h a r a c t e r i s e d in that the polymer composition further contains a conven¬ tional silanol condensation catalyst selected amongst car¬ boxylic acid salts of the metals tin, zinc, iron, lead and cobalt; organic bases; inorganic acids; and organic acids.

9. The cross-linkable polymer composition of any one of the preceding claims, c h a r a c t e r i s e d in that the acid anhydride catalyst constitutes about 0.01-1.5% of the total weight of the polymer composition.

10. The cross-linkable polymer composition of any one of the preceding claims, c h a r a c t e r i s e d in that the boiling point of the acid anhydride catalyst is at least about 120°C.