GB2296718A - Infrared-activated curing of polysiloxane compositions - Google Patents

Description

2296718 METHOD FOR INFRAREDACTIVATED CURING OF POLYORGANOSILOXANE

COMPOSITIONS

Background of the Invention

This invention relates to the infrared-activated curing of organopolysiloxane compositions, and more particularly to curing by hydrosilylation.

The curing of organopolysiloxane compositions (hereinafter sometimes simply "silicones") by hydrosilylation is disclosed in numerous patents and publications. It involves the reaction, generally catalyzed by a platinum group metal-containing catalyst, of an alkenyl-substituted silicone with a silicone containing at least one Si-H bond. The cured material is generally a crosslinked silicone rubber.

While many hydrosilylation reactions are promoted simply by conventional heating of the curable compositions, other types of activation by heating are known. For example, Japanese Kokai 571110,433 and copending, commonly owned application EP-A-0597613 disclose the use of infrared radiation to heat the silicone materials. As disclosed in said copending application, infrared curing takes place in the presence of a suitable infrared absorbing or scattering material and typically employs radiation of wavelengths in the range of 700-10,000 nm.

As described in European patent application 358,452, hydrosilylation may also be effected by exposure of the curable composition to visible light in the presence of a sensitizer. Curing by this method apparently takes place at ambient temperature, since no heating temperature is specified. The wavelength of the visible light employed is stated as being about 400800 nm. However, those skilled in the art recognize that radiation with a wavelength greater ID-2281 0 than 700 nm. (7,000 A) is more accurately in the infrared region, in accordance with CRC Handbook, 65th Edition (1984), page E-184.

Uniform curing of hydrosilylation mixtures requires rapid, uniform volumetric heating. This in turn requires uniform absorption of infrared radiation through the bulk of the resin being cured. When infrared radiation in the above-described broad wavelength range is employed at high intensity, it is frequently found that the silicone composition begins to smoke before curing is complete, indicating that surface decomposition has begun. Such decomposition generally occurs at temperatures above about 225C and especially above 250C.

It is desirable, therefore, to develop an improved method for infrared heating of hydrosilylation compositions, to facilitate uniform cuhng thereof without decomposition. Such a method is provided by the present invention.

Summary of the Invention

The invention is a method for curing by hydrosilylation a composition comprising (A) an alkenyl group-substituted polyorganosiloxane, (B) a polyorganosiloxane having at least one SM bond and (C) at least one infrared absorbing material. Said method comprises heating said composition to a temperature in the range of 100- 210C by infrared radiation having a wavelength limited to the range of 700-3,000 rim.

Detailed Description; Preferred Embodiments

Reagent A according to the present invention is at least one alkenyl-substituted polyorganosiloxane. It is preferably one having vinyl groups bonded to silicon. Such silicone materials are well known in the art and have been employed previously in the preparation of cured silicone materials, including foams. They are described, for example, in U.S. Patents 4,418,157, 4,851,452 and D-2281 0 5,011,865, the disclosures of which are incorporated by reference herein.

A typical linear (polydiorganosiloxane) silicone material useful as reagent A is represented by the formula (1) W.

R' R si-o 01-R n R wherein each R1 is independently C1-6 alkyl, phenyl, 3,3,3trifluoropropyl or vinyl and n has a value such that the viscosity of the silicone is in the range of about 100-1,000,000, preferably about 1,000-250,000 and most preferably about 2,500-100,000 centipoise at 25'C. Most often, each R1 that is not vinyl is methyl.

An art-recognized convention for designating silicone structural units in accordance with the number of oxygen atoms attached to silicon is employed herein. That convention uses the letters M, D, T and 0 to designate said number of oxygen atoms as abbreviations for Omono", "di", "tri," and "qualtro". Thus, the silicone of formula I consists of M end groups and D internal units. The presence of T and/or Q units imparts branched and/or crosslinked structure to the compound.

The proportion of M, D, T and Q units in reagent A and in the mixture as a whole may be varied to afford a composition of the desired degree of branching and other properties. Thus, for example, the aforementioned U.S. Patent 4,418,157 describes a base silicone material which may contain vinyl groups bonded to silicon and which has prescribed proportions of M, D and 0 units.

In general, reagent A comprises principally compounds in which vinyl groups are bonded to terminal silicon atoms on the silicone chain.

D-2281 0 of the formula Reagent B may be represented by a linear polysiloxane R 2 R 2 1 1 S R 2 S,__0 51-R 2 2 n R 2 1 wherein each R2 is independently Cl -6 alkyl, phenyl, 3,3,3- trifluoropropyl or hydrogen. Most often, reagent B has an average of at least about three Si-H moieties per molecule and an average of no more than one hydrogen atom bonded to any silicon atom, and any non-hydrogen R2 values are methyl.

Reagent C is at least one Infrared radiation absorbent or scattering material. Suitable materials of this type include inorganic materials such as carbon blacks and graphites, cerium oxide, titanium oxide and iron (111) oxide; ceramics such as porcelain; infrared absorbing pigments such as Prussian blue; organometallic compounds such as (methylcyclopentadienyl) manganese tricarbonyl and (tetraphenylcyclobutadiene)(cyclopentadienyi)cobalt; and organic compounds such as anthracene, phenanthracene, anthraquinone and phenanthracenequi none. The inorganic materials are generally preferred, with oxides and salts often being most preferred.

In general, the method of this invention will also employ (D) at least one hydrosi)ylation catalyst, most often a platinum group catalyst. By "platinum groupo is meant the portion of Group VIII of the Periodic Table, as traditionally identified, containing the metals rhodium, ruthenium, palladium, osmium, iridium and platinum. The preferred metals from this group are rhodium, palladium and platinum, with platinum being particularly preferred because of its relative availability and particular suitability.

D-2281 0 Numerous types of platinum catalysts are known in the art and are disclosed in the patents incorporated by reference hereinabove. They include, for example, reaction products of chloroplatinic acid with olefins, alcohols, ethers, aldehydes--and vinylsiloxanes such as tetramethyldivinyldisiloxane. A reaction product of chloroplatinic acid with tetramethyldivinyldisiloxane in the presence of sodium bicarbonate as disclosed in U.S. Patent 3,775,452, dissolved in xylene to a level of about 5% by weight platinum, is preferred; it is hereinafter designated "Karstedt's catalyst".

Other materials such as desiccants and fillers may also be present. Suitable fillers include reinforcing fillers such as fumed silica and precipitated silica and extending fillers such as ground quartz, titanium dioxide, zinc oxide. zirconium silicate, silica aerogel, iron oxide, diatomaceous earth, calcium carbonate, magnesium oxide, calcined clay and carbon (e.g., graphite or carbon black). Silica, and especially fumed silica, is often preferred. It will be apparent that some of these fillers are also useful as infrared absorbing materials.

The proportion of reagent B is most often about 0.5-50, preferably about 10-20, parts by weight per 100 parts of reagent A. Infrared absorbing materials are employed in an effective amount to absorb the incident radiation; said amount may vary widely depending on the type of material used, but is usually in the range of about 0.0001-10.0 parts per 100 parts of reagent A. When employed, fillers are typically present in the amounts of about 15-50 parts respectively per 100 parts of reagent A.

Any amount of catalyst (reagent D) effective to catalyze hydrosilylation of reagent A with reagent B may be employed. Typically, an amount to provide about 10-100 and preferably about 10-50 parts by weight of platinum per million parts of total composition is employed.

D-2281 0 According to the present invention, the silicone composition is heated to a temperature in the range of 100-21 OC by impinging thereon infrared radiation in the reduced wavelength range of 700-3,000 nm. The radiation may be produced by conventional infrared radiation sources such as tungsten-halogen lamps. Power level will depend on such factors as lamp- to-silicone spacing and the size of the silicone article being produced; typical power levels are in the range of about 100-5,000 watts.

Screening of radiation in the wavelength ranges not desired may be achieved by methods known to those skilled in the art. For example, a shield of borosilicate glass will absorb radiation at 3000 nanometers and upward. To remove radiation below 700 nm, a it cool mirror" may be employed. An example of such an item is available from Edmund Scientific Company as part number R42,414.

The cool mirror is typically located between the borosilicate glass shield and the silicone composition.

The effect of infrared shielding and limitation of the incidence radiation to the 700-3,000 nrn wavelength range is illustrated in a series of experiments in which a liquid silicone injection molding composition was employed both in curable and in control form. The alkeny ksubstituted polyorganosiloxane constituents therein were chiefly provided by "Masterbatch A" which was a mixture of 90% (by weight) vinyl-terminated polydimethylsiloxane having a viscosity of 40, 000 centipoise, 5% vinyl-terminated polydimethylsiloxane having a viscosity of 4,000 centipoise (hereinafter "4,000 centipoise vinylsilicone") and 5% polydimethylsiloxane having terminal and internal vinyl groups and having a viscosity of 500 centipoise. 'Masterbatch B" was prepared by blending about 80 parts of Masterbatch A with about 20 parts of fumed silica having a surface area of 200 m2/g.

To prepare the control composition, 92 parts by weight of Masterbatch B was blended with 4 parts of a composition comprising 50% by weight of the 4,000 centipoise vinyisilicone, 25% D-2281 0 cerium(IV) oxide and 25% magnesium oxide, and 4 parts of a composition comprising 50% of the 4,000 cenUpoise vinylsilicone and 50% zinc oxide. Thus, the control composition contained 1.3 parts by weight of reagent C and 27.6 parts of filler (silica, magnesium oxide and zinc oxide) per 100 parts of reagent A.

A sample of the control composition, 4 mm. thick, was placed 8 cm. below a 600-watt tungsten halogen lamp. Three thermocouples were vertically placed in the silicone sample, 1 mm.

below its surface and at 1 -mm. spacings, and the lamp was activated.

After 2 minutes, smoke emission at the surface of the silicone composition was observed although the temperatures at 1, 2 and 3 mm. below the surface were only about 120', 110 and 85C respectively.

The procedure was repeated with a borosilicate glass slide interposed as a shield between the lamp and the silicone composition and a cool mirror between the glass slide and the silicone. Thus, the wavelength of radiation incident on the silicone was from 700 to 3,000 nm. After about 2.5 minutes, the temperatures 1, 2 and 3 mm. below the surface were about 150, 145 and 130C respectively and no surface smoke generation was observed. Thus, heating was much more uniform when the higher wavelength radiation was screened out.

The procedure with the borosilicate slide and "cool mirror" was repeated with a composition prepared by employing 46 parts each of two vi ny Isubstituted polydimethylsiloxane blends, the first additionally containing 2 parts of a methyl-substituted, hydrogen endstopped MO resin and about 0.7 part of a MIDIV1 resin containing about 30 Si-H moieties per molecule and the second additionally containing 26 ppm of platinum as Karstedt's catalyst. Thus, the composition contained 1.6 parts of reagent B per 100 parts of reagent A and a total of 12 ppm of platinum. Upon curing with infrared radiation in the 700-3,000 nm range, a fully cured silicone rubber was produced after about 2.5 minutes.

lb-2281 0

Claims (14)

1. A method for curing by hydrosilylation a composition comprising (A) an alkenyl group-substituted polyorganosiloxane, (B) a polyorganosiloxane having at least one Si-H bond and (C) at least one infrared absorbing material, said method comprising heating said composition to a temperature in the range of 100-210C by infrared radiation having a wavelength limited to the range of 700-3,000 nm.

2. A method according to claim 1 wherein the alkenyl groups are vinyl.

3. A method according to claim 2 wherein said composition also contains (D) at least one platinum group hydrosilylation catalyst.

4. A method according to claim 1 wherein the proportion of reagent B is about 0.5-50 parts by weight per 100 parts of reagent A.

5. A method according to claim 3 wherein each organo group in reagent A which is not vinyl is methyl and each organo group in reagent B is methyl.

6. A method according to claim 5 wherein reagent A comprises principally compounds in which vinyl groups are bonded to terminal silicon atoms on the silicone chain.

7. A method according to claim 3 wherein the platinum group metal in reagent E is platinum.

8. A method according to claim 7 wherein reagent E is a reaction product of chloroplatinic acid with tetramethyidivinyidisiloxane.

9. A method according to claim 8 wherein the proportion of platinum is about 10-50 parts per million parts of total composition.

D-2281 0 silica.

10. A method according to claim 3 wherein the proportion of reagent C is in the range of about 0.0001-10.0 parts per parts of reagent A. -

11. A method according to claim 3 wherein reagent C is at least one inorganic oxide or inorganic salt.

12. A method according to claim 3 wherein said composition also contains at least one filler.

13. A method according to claim 12 wherein the proportion of filler is about 15-50 parts respectively per 100 parts of reagent A.

14. A method according to claim 13 wherein said filler is