CA1140693A

CA1140693A - Reducing set of organopolysiloxane elastomers with polycarbodiimide-polysiloxane copolymers

Info

Publication number: CA1140693A
Application number: CA000338810A
Authority: CA
Inventors: Werner Michel; Wilfried Kniege; Helmut Steinberger; Hans-Heinrich Moretto
Original assignee: Bayer AG
Current assignee: Momentive Performance Materials GmbH
Priority date: 1978-11-02
Filing date: 1979-10-31
Publication date: 1983-02-01
Also published as: JPS595620B2; JPS5562961A; EP0010708B1; EP0010708A1; DE2847481A1; DE2963509D1

Abstract

REDUCING SET OF ORGANOPOLYSILOXANE ELASTOMERS
WITH POLYCARBODIIMIDE-POLYSILOXANE COPOLYMERS

ABSTRACT OF THE DISCLOSURE

In polymerizing organopolysiloxanes to elastomers, the permanent set of the product is reduced by incorporating therein polycarbodiimide-polysiloxane copolymers along with the conventional catalysts and fillers.
Le A 19 084

Description

~ - ~ ~
6~3 Reducing sèt o~ organopolysiloxane elastomers with polycarbodiimide-polysiloxane copo~mers The invention relates to a process for the prepara-tion of compositions, based on organopolysiloxanes, which can be cured to give elastomers having reduced permanent set.
Conventional organopolysiloxane elastomers, when tested according to DIN 53,517, can have a permanent set of 95% when exposed ~or some time to temperatures of above lo 150C. Ho~ever, for many applications, for example as gaskets, it is necessary that organopolysiloxane elas-tomers should be capable of exposure to temperatures of up to 300C or more for long periods without an excessive increase in the permanent set.
Hitherto, compositions, heat-curable to give elastomers, which are based on organopolysiloxane composi-tions with metal oxides, metal peroxides or metal carbon-ates as agents for reducing the permanent set have been dis-closed. Inter alia, the relevant compounds of barium, 20 cadmium, magnesium, praseodymium and strontium have been proposed for this purpose. According to U.S. Patent Specification 2,448,530, mercury, its salts or oxides are added to the silicone rubber, before vulcanization in order to reduce the permanent set. However, the high toxicity 25 of the mercury preparations prevents a general applic-ability of this process. According to U.S Patent Specification 2,713,564, alkyldithiocarbamates of sodium, zinc, lead, tellurium, selenium, copper or bismuth can take the place of the mercury compounds. Fur-thermore, accord-ing to U.S. Patent Specification 2,666,041 the addition of certain quinones such as, for example, naphthoquinone, alkylated quinones, halogenated quinones or ~ydrocarbon esters of hydroquinone have an advantageous effect in reducing the permanent set of silicone rubber.

Le A 19 084 A great advance was achieved with the introduction of silanes, con-taining vinyl groups, into silicone rubber technology; these compounds, in con-junction with curing agents based on alkyl peroxides or alkyl per-esters and in conjunction with the metal-catalyzed addition crosslinking of hydrogenosiloxanes to vinylsiloxanes permit the preparation of vulcanized products having low permanent set.
Nevertheless, even in the case of silicone rubber mixtures containing vinyl groups the permanent set of the products ob-tained by crosslinking with acyl peroxides, especially bis-(2,4-dichlorobenzoyl) peroxide, remains unsatis-factorily high. An explanation of this phenomenon cannot be provided. In spite of this disadvantage, bis-(2,4-dichlorobenzoyl) peroxide cannot be replaced in every case by the systems mentioned above since, for example, alkyl peroxides show unsatisfactory crosslinking characteristics in the continuous vulcanization of endless strands, cables or hoses, if the vulcanization is carried out without application of pressure, in hot air or by means of infra-red radiation. The addition crosslinking method cannot yet be regarded as industrially fully developed.
Continuous vulcanization, wlthout using pressure, b~ means of hot air or infra-red radiation has found general acceptance for the vulcanization of ~0 silicone rubber in preference -to other processes such as the salt bath or steam tunnel process. In this type of vulcanization has hitherto been necessary to accept a high permanent set, resulting from the peculiarity of the bis-(2,4-dichlorobenzoyl) peroxide. The present invention now rela-tes to compositions, based on organopolysiloxanes, which can be cured to give elastomers having reduced permanent set, characterized in that polycarbodiimide in the form of a polycarbodiimide-polysiloxane copolymer, preferably in an amount of about 0.1 to about 8~ by weight, based on -the to-tal mixture is added to a mixture consisting of

2 --~,_ . .

6~3 a~ an organopolysiloxane polymer having a viscosity of 1 000 000 to 200,000,00o mPas (at 25C) and co~prising the structural units (R)aSiO4-a The radicals R in the organopolysiloxane polymer or the mixtures of such organopolysiloxane polymers and especially in a diorganopolysiloxane polymer having a vis-10 cosity of about 1,000,000 to 200,000,000 centipoise at25C are selected for example, from amongst monovalent hydrocarbon radicals, halogen-substituted monovalent hydrocarbon radicals and cyanoalkyl radicals. Such radicals are, for example alkyl radicals, such as methyl, 15 ethyl and propyl, cycloalkyl radicals, such as cyclohexyl and cycloheptyl, alkenyl radicals, such as vinyl and allyl, halogen-substituted alkyl radicals, such as fluoro-propyl and trifluoropropyl and in particular fluorinated alkyl radicals of the formula R4CH2-, R4 being a perfluoro-20 alkyl radical, mononuclear aryl radicals, such as phenyl,alkaryl radicals, such as methylphenyl and ethylphenyl, aralkyl radicals such as phenylmethyl and phenylethyl, cyanoalkyl radicals, such as cyanopropyl and the like, as well as other substituents which are usually encountered `25 as substituents of linear diorganopolysiloxanes. It is particularly preferred to select the radicals R from amongst alkyl radicals with 1 to 8 carbon atoms, alkenyl radicals with 2 to 8 carbon atoms, halogen-substituted alkyl radicals, such as fluoroalkyl radicals 30 with 3 to 8 carbon atoms, and mononuclear aryl radicals.

Le A 19 o8 4 6~:~3 a has a value between about 1.95 and 2.01, b) curing catalysts, c) fillers and d) further additives which are in themselves known.
Further additives used as a rule are pigments, anti-oxidants and hot air stabilizers based on known metal oxides. Examples of reinforcing fillers are, in particular, silicon dioxide produced pyrogenically in the gas phase, precipitated silicon dioxide having a surface area of 10 at least 50 m2/g, and silicic acid hydrogels dehydrated in such a way as to retain the structure. Examples of ~on-reinforcing fillers are, in particular, diatomaceous earth, ~uartz powder and chalk. Titanium dioxides,iron oxide, Al203, silicates and the like are also suitable.
Examples of curing agents are alkyl peroxides, aryl peroxides or acyl peroxides, used individually or in combination. However, the organopolysiloxane compositions can also be cured by gamma-rays.
The specific peroxide curing catalysts which are 20 preferred include di-tertiary-butyl peroxide, tertiary-butyl triethylmethyl perioxide, tertiary-butyl triphenyl-methyl peroxide, tertiary-butyl perbenzoate and di-terti-ary-alkyl peroxides, such as dicumyl peroxide. Other suit-able peroxide catalysts which cause curing both via 25 saturated and via unsaturated hydrocarbon groups on the silicone chain are aryl peroxides, the benzcyl peroxides, mixed alkylaryl peroxides, such as tertiary-butyl perbenzoate, chloroaroyl peroxides. such as 1.4-dichlorobenzoyl peroxide. 2.4-dichlorobenzoyl 30 peroxide and monochlorobenzoyl peroxide, benzoyl-peroxide methyl ethyl ketone peroxide and the like. ~n-general, 0.1 to 8% by weight of the peroxide, relative to the rubber, are employed. Preferably, about 0.5 to 4~ by weight are employed.

Le A 19 084 ~Q~3 Surprisingly, the use of polysiloxanes containing polycarbodiimide leads to an unexpected solu-tion of -the problem. If the polycarbodiimide-polysiloxane copolymer (polycarbodiimide is usually about 3-80% by weight, preferably about 5-7% by weight, of the polycarbodiimide-polysiloxane copolymer) is added in amounts of about 0.1 to 12% by weight, relative to the total polymer, preferably in amounts of about 0.5 to 3% by weight, to a customary silicone rubber mixture, the permanent set of the vulcanized products is substantially reduced.
The polycarbodiimide-polysiloxane polymers are characterized in that the polysiloxane and polycarbodiimide are present as distinguishable phases, optionally with partial chemical and/or physical bonding to one another. They are polycarbodiimide-filled polysiloxanes which are composed of the following two phases: (i) a continuous phase consisting of an organopolysiloxane liquid and (ii) a discontinuous phase consisting of finely dispersed particles of a carbodiimide polymer, which has been obtained by polycondensation of a corres-ponding monomer or monomer mixture in the presence of the oryanosiloxane liquid described, and of a carbodiimidation catalyst.
These polycarbodiimide-filled crganopolysiloxane compositions are in themselves known and their preparation is described in detail in our Canadian ~0 Application No. 306,852, filed July 5, 1978. The form in which the polycarbodi-imide polysiloxane polymer is introduced into the mixture is immaterial. For example, the polymers can be premixed with one another and then with fillers and auxiliaries, after which the polycarbodiimide-polysiloxane polymer is ad mixed; alternatively, a premix of a customary polysiloxane polymer with fillers and auxiliaries is blended with the polycarbodiimide polymer, or the poly-carbodiimide polymer is premixed with fillers and auxiliaries and blended or :; ~

mixed with a polysiloxane, or premixes of both types of polymers are blended with one another in the desired ratio.
The mixing temperature is not subject to any particular limitations. All procedures customary for silicone rubber can also be employed in the present case.
Equally, all processes and agents customary in the tech-nology of silicone rubber manufacture can be used.
For example, the individual components of which the rubber mixture consists can be mixed with one another at any temperature above the glass transition temperature of the polymer, for example at room temperature, and during the mixing process the temperature of the mixture may rise to 70 - 100C; alternatively, the components can be mixed by means of heated mixing equipment at temperatures of up to 200C. Further customary methods are those in which the miJYture is first prepared at temperatures below 100C
and is then subjected to an after-treatment at temperatures above 100C, for example 150C. The abovementioned mixing process can also take place under reduced pressure in order to remove undesired volatile constituents from the mixture.
The mixing can be carried out on the open two-roll equipment customary in the rubber industry, and this equip-ment can be cooled or heated as desired. Further, paddle kneaders of the type o~ the sigma-kneaders or Z-blade kneaders, without floating weights, and with or without a discharge screw, and floating-weight kneaders of the Banbury type, as well as continuous mixing equipment, for example twin-screw mixers and single-screw mixers, can be used.
The advantageous effect o~ the polycarbodiimide polymer on the permanent set is particularly marked in the case of those vulcani2ed silicone rubbers where the perman-ent sèt is high. However, even in the case of rubber mixtures which gi`ve vulcanized products of low permanent set, a further improvement as a result of the additive Le A 19 084 (?~93 according to the invention can be observed, There follow examples o~ the preparation of silicone elastomers, which illustrate the present invention in more detail. Mixtures of the abovementioned type were pro-duced under the customary conditions in a mixer, and testspecimens were prepared therefrom. The ~ulcanization of the test specimens was carried out in a heated press.
The permanent set was tested in accordance with DIN speci-~fication 53,517. The composition of the samples is given in parts by weight.
Example 1 This example illustrates a method of preparation o~ the polycarbodiimide-polysiloxane copolymer which is employed in rubber mixtures.
For this preparation, 20 kg o~ a polydimethylsilox~
ane with terminal hydroxyl groups, having a viscosity o~
18,000 mPas, are stirred by means of a stirring disc at 500 to 800 rpm, and warmed to 70C, 30 g of a l-methylphos-pholine oxide isomer mixture are added and 20 kg of an isomer mixture of 80 per cent by weight of toluylene-2,4-diisocyanate and 20 per cent by weight of toluylene-2,6-diisocyanate are metered into this mixture in a uniform stream over the course of 2 hours, with constant stirring.
The carbon dioxide ~ormed is led away. A~ter completion ~5 of the addition o~ the isocyanate, stirring is continued for one hour at the same temperature, a~ter which the product is cooled to room temperature.
The product is a white to pale yellowish, viscous composition having a viscosity of about 300,000 mPas.
30 Example 2 This example describes the prepara-tion of a poly-carbodiimide-polysiloxane copolymer based on diphenyl-methane-4,4'-diisocyanate~
1,500 g of polydimethylsiloxane with terminal 35 hydroxyl groups and having a viscosity o~ 18,000 mPas are Le A 19 08~

heated to 80C and 1.5 g of l-methylphospholine oxide iso-mer mixture are added. 1,500 g of diphenylmethane-4,4'-diisocyanate, which has been warmed to 40C, are metereà
in over the course of 2 hours at 80C, with constant stirring by means of an anchor stirrer, and a~ter comple-tion of the addition stirring is continued for 1 hour at 80C. A white, pourable composition having a viscosity of 330 000 mPas is obtained.
Examples 3 to 6 A silicone rubber premix is prepared on a rubber mixing mill by mixing, in the usual manner, 27.5 parts of a pyrogenically produced silica having a surface area of at least ~00 m2/g and 6.33 parts of a silanol-based processing auxiliary into 100 parts of-a polydimethylsilcxane containing vinyl 15 grcups (~iscosity 4.5i10 mæas, methylvinylsiloxy content 0.175 mol~
This premix is divided into 4 equal parts and each is mixed, on the rubber mixing mill, with ~iatomaceous earth and dicumyl peroxide and come with a polycarbodi-imide-polysiloxane copolymer ac~cording to Example 1, 20 in accordance with Table 1 below.
Sheets of 2 and 6 mm thickness of these four mix-tures were vulcanized in a heated press at 170C. The vulcanization time is 10 minutes. The test specimens according to DIN specification 539517 are cut from these 25 sheets.
Some of the samples are aged, after vulcanization, for 6 hours in hot air at 200C~ Thereafter, the perman-ent set is tested in accordance with the abovementioned DIN specification.

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Examples 7 to 10 A silicone rubber premix is first prepared on a rubber mixing mil1 by mixing, in the usual manner, 100 parts of a polydimethylsiloxane containing methyl vinyl siloxy groups (0.125 Mol%, 4,5,106 mPas) 27.5 parts of a pyrogenically produced silica having a ~ET surface area of at least 300 m2/g and 6.33 parts of an organo-silanol-based processing auxiliary. This premix is divided into 4 equal parts and each of these is mixed 10 in accordance with the numerical data in Table 2 below, with diatomaceous ear~h and a 50 per cent strength paste of bis-(2,4-dichlorobenzoyl) peroxide in silicone oil, and in three instances with a polycarbodiimide-polysiloxane copolymer according to Example 1.
These 4 mixtures were used, on the one hand, to vulcanize sheets of 2 and 6 mm thickness for 10 minutes in a heated press at 120C, some of the sheets then being treated in hot air at 200C for 6 hours; on the other hand, some sheets were not treated in a press and 20 only exposed to hot air at 200C for 10 minutes. Following the w lcanizationj the permanent set is determined in accordance with DIN specification 53,517.

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Le A 19 084 Exam~les 11 and 12 A silicone rubber premix as described in Examples

3. to 10 is ~irst prepared on a rubber mixing mill.
60 parts of diatomaceous earth and 1.5 parts of a 50 per cent strength paste of bis-(2,4-dichlorobenzoyl) peroxide in silicone oil are added to 100 parts of this premix.
The sample is divided and one half is left unmodified.
1 part of polycarbodiimide-polysiloxane copolymer of Example 1 is added to 100 parts of the other half, and mixed in.
2 and 6 mm sheets produced from a part of both samples are vulcanized in a heated press at 120C. The vuIcanization time is 10 minutes. Some of the vulcan-ized sheets are treated in hot air at 200C. The dura-tion of the hot air treatment is 6 hours.
2 and 6 mm thick sheets were also produced fromanother part of both samples, and these sheets were vulcanized by hot air at 200C over the course of 10 minutes.
Thereafter, the permanent set is determined in accordance with DIN specification 53,517.
Table 3 Perm~nent set (%) after w lcanization 10'/120C 10'/120~C 10'/200C
+6 hours/200C in hot air in hot air . . _ _ ~xample 11 29.7 15.4 27.4 Example 12 16.5_ 6.0 16.4 Examples 13 and 14 A silicone rubber premix is prepared on a rubber mixing mill by mixing 100 parts of a polydimethylsiloxane containing vinyl groups, 1 part of hexamethyldisilazane, 58 parts of pyrogenically produced silica having a sur~ace area of 200 m2/g and 12.76 parts of an organosilanol-based processing auxiliary. The premix is divided and the first hal~ (Example 13) is mixed with 1.4 parts of bis-Le A 19 084 .. .. . . . . . .. . .... . ..

6~33 (2,4-dichlorobenzoyl) peroxide, and the second half (Example 14) with 1.4 parts o~ bis-(2,4~dichlorobenzoyl) peroxide and 6 parts of polycarbodiimide-polysiloxane copolymer according to Example 2.
Sheets o~ 2 and 6 mm thickness are prepared from both mixtures and are vulcanized over the course of 10 minutes at 120C. The permanent set determinations according to DIN 53,517 are shown in Table 4.
Table 4 Permanent set (%) after vulcanizationl (lOt/120C) 0 Example 13 Example 14 _ 81~9 57.1 It will be appreciated th~t the instant specification and examples are set forth by way of illustration and not 15 limitation, and that various modifications and changes may be made without departing from the spirit and scope of the prasent invention.

Le A 19 084 .. ..

. .

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A composition which is heat-curable to give an elastomer of reduced permanent set comprising:
(a) an organopolysiloxane polymer having a viscosity of 1,000,000 to 200,000,000 mPas at 25°C and comprising the structural unit wherein R is a hydrocarbon radical or a halohydrocarbon radical, and a is between about 1.95 and 2.01;
(b) a polycarbodiimide-polysiloxane copolymer;
(c) a curing catalyst; and (d) a filler.

2. A composition according to claim 1, in which R is methyl or vinyl.

3. A composition according to claim 1, in which polycarbodiimide is from 3 to 80% by weight of the polycarbodiimide-polysiloxane copolymer and the poly-carbodiimide-polysiloxane copolymer is from about 0.1 to 12% by weight of the total composition.

4. A composition according to claim 2, in which polycarbodiimide is from 5 to 70% by weight of the polycarbodiimide-polysiloxane copolymer and the poly-carbodiimide-polysiloxane copolymer is from about 0,05 to 3% by weight of the total composition.

5. A composition according to claim 1, 2 or 3 wherein the polycarbodi-imide-polysiloxane copolymer is from about 0.1 to 8% by weight of the total composition.

6. In the production of an elastomer by heat curing a composition com-prising an organopolysiloxane, a curing catalyst and a filler, the improvement which comprises incorporating in such composition a polycarbodiimide-poly-siloxane copolymer whereby the permanent set of the elastomer is reduced.