CA1133160A - Silicone dental impression compositions - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Two-part room temperature vulcanizing silicone dental impression compositions are provided which, before curing, consist essentially of a metallic salt of a monocarboxylic acid as a catalyst and a base compound.
In one embodiment the base compound consists of a fluid diorganopolysiloxane, an organolsilicon cross-linker and a unique filler combination of an admixture of zinc oxide, calcium carbonate and pumice. In another embodiment, the base compound consists of a fluid diorganopolysiloxane, an organosilicon cross-linder, mineral oil and a low oil absorption calcium carbonate filler.

Description

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IMPROVED SILICONE DENTAL IMPRESSION COMPOSITIONS
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This application is a division of Canadian Application Serial Number 254,987, filed June 16, 1976.
The present invention relates to improve silicon compositions for making detailed impressions of the mouth, gums and - teeth. More particularly, it concerns two-part room-temperature vulcanizing silicone 10 compositions, one part being a metallic salt of a monocarboxylic acid as a catalyst .
and the second part being the base compound.
In one embodiment, the base compound consists of a diorganopolysiloxane, an organosilicon for cross-linking and a new improved filler, which is an admixture of zinc oxide, calcium carbonate and pumice. In a second embodiment, the base compound consists of a diorganopoly-siloxane, an organsilicon cross-linker 20 mineral oil and, as an improved filler, a low oil absorption calcium carbonate.

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BACKGROUND OF T~E INVENTION
Room-temperature-vulcanizing silicone rubber has enjoyed increasingly wider use in dentistry as a molding medium in the preparation of artificial dentures.
The material is used as a paste into which a curing agent is mixed before use. It then changes from a pasty consistency to a rubber-elastic state upon curing at body temperature. Because of its elasticity, the cured silicone can be removed from the mouth easily and can then be used for casting a working model, such as with plaster of paris, from which the permanent dental piece is made.
Silicone rubber compositions offer several advantages over materials formerly used as a dental replica material, e.g., plaster, alginate, etc. As compared with plaster, silicone rubber is more elastic, has better resistance to breakage and offers good release from the jaw. As compared with alginates, silicone rubber is not sensitive to loss of water (which may cause shrinkage) and possesses better molding accuracy. Silicones also provide good dimensional stability which is retained even on prolonged storage in ~he air ~For reference, see "Chemistry and Technology of Silicones", Walter Noll~ Academic Press, New York, 1968, p. 623.~
Elastomeric compositions have been utilized in two main areas of dental impression work - one, crown and bridge work and, two~preparation of ~ull dentures. Crown and bridge work requires a custom tray which is usually prepared by first making an impression with alginate of the area of the mouth to be repaired. A cast of this impression is then made by ~ 8SI-1384A

pouring dental stone into the hardened alginate and curing it. The cast is then wrapped with asbestos and a form is built up around the wrapped casting using methyl methacrylate or some other suitable polymeric material (e.g.) polyethylene, polyvinyl chloride, polyepoxide, etc.). After the methyl methacrylate has been cured, it is removed from around the dental stone casting and used as the custom tray for the silicone rubber impression that will be made.
The methyl methacrylate custom tray is first coated with a primer. Then a silicone of pasty consistency is mixed with a curing agent and some suitable catalyst, placed into the tray, and the tray is inserted into the patient's mouth where curing takes place. After this initial impression has cured, it is removed and trimmed to eliminate undercuts and thin-walled sections. Then a silicone compound of lower viscosity is catalyzed and inserted into the patient's mouth directly over the area to be reparied.
The custom tray containin~ the cured silicone rubber initial impression is pressed firmly into place over the catalyzed silicone until curing is complete. This part of the procedure is then finished, and now highly detailed impression of the patient's mouth is forwarded to a laboratory for the making of a bridge.
For the preparation of full dentures, the silicones are utilized in much the same manner, except that a reusable metal perforated tray is used instead of a custom tray made of polymeric materials. The high viscosity silicone compound is placed into the tray as before, inserted into the mouth to make a first ~ 8SI~1384A

impression, and this is followed by the use of a lower viscosity silicone compound to make a more detailed impression of the area of the mouth to be repaired, st~stantially as described above.
An elastic silicone composition which can be used to obtain room temperature vulcanizing dental impressions as described above is disclosed in Nitzsche et al, U.S. Patent No. 3,127,363, issued March 31, 1964.
Nitzsche et al disclose organopolysiloxane elastomeric compositions that are vulcanizable at room temperature and are taught as being useful for a variety of purposes, including the making of casts of the mouth and teelh. However, these compositions are not specifically adapted to some of the requirements relating uniqueIy to the making of dental impressions.
Lampe, U.S. Patent No. 3,6~6,090, issued October 3, 1972, di~scloses a room temperature ctlrable silicone rubber composition which can be used making ear plugs. However, relatively small amounts of filler (10-50%~ must be employed or the composition is difficult to mold.
There have now been discovered new improved two-part silicone rubber compositions which are vulcanized at room temperature and are adapted for making dental impressions. The present compositions provide desired properties such as moderately fast "pot life", putty-llke consistency, good shelf stability, low linear shrinkage, and low-toxicity.
Vulcanization of the compositions of the present invention is not inhibited by materials from which the custom trays are made, e.g., polyarcylics, polyethylene, polyvinylchlorides, epoxies, and the like.

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The compositions achieve greater hardness (duro-meter build-up) within a shorter time after cure, and can be formulated to cure to a smooth, non-grainy appearance.
In addition, the need for custom tray fabrication by the dentist is eliminated because the compositions of the present invention can be employed both as the custom tray material and as the high viscosity compound which is inserted into the tray to make dental impressions. ~oreover, bit registration, which usually requires a separate operation with either a custom tray made of methyl methacrylate or the like or a metal perforated tray, can now be obtained with the present silicone putty tray material at the same time as the dental impression.
The detailed compositions are not disclosed in the pripr art, and the remarkable properties, as ; dental impression materials, are not obvious from anything disclosed or remotely suggested in the prior ........ .
art.
In accordance with one aspect of the invention there is provided a two-part room temperature vulcani2ing silicone dental impression composition which, before curing, consists essentially of from about 0.3 to about 0.7 parts by weight of a metallic salt of a monocarboxylic acid as a catalystt to about 100 parts of a base compound consisting essentially of:

(a) from about 25 to about 35~ by weight of a fluid diorganopolysiloxane containing terminal silicon-bonded hydroxy groups and having a viscosity of from 2,000 to 250,000 cps. at 25C.;
(b) from about 60 to about 75% by weight of 8SI-138~A

a fillex composition consisting essentially ~f an admixture of zinc oxide, calcium carbonate and pumice;
(c) from about 0.05 to about 2% by weight of an organosilicon cross-linker having the general formula OR
RO - Si ~ R
O~
wherein R is a radical selected from the group consisting : 10 of alkyl, alkenyl and aryl xadicals and R] i~ a member of the group consisting o~ alkyl, alkenyl, aryl and alkoxy radicals.
In accordance with another aspect of the invention there is provided a room temperature vulcanizing silicon dental impression composition which, before curing, consists essentially of from about 0.3 to about 0O7 parts by weight of a metallic salt of a monocarboxylic acid as a catalyst to about l00 parts of a base composition consisting èssentially of:
(a) from about 25 to about 35% by weigh~
of a fluid diorganopolysiloxane containing terminal silicon-bonded hydroxy groups and having a viscosity of rom 2,000 to 250,000 cps. at ~5C.;
(b) from about 63 to about 75% by weight o~
a filler composition consisting essentially of an admixture of zinc oxide, calcium carbonate and pumice, .
the zinc oxide and calcium carbonate each being present in at least a sufficient amount to provide bulking and whitening and said pumice being present in at least a sufficient amount to provide putty-like consistency;
(c) from about 0.05 to about 2% by weight of an organosilicon cross-linker having the general ~33~ 8SI-1384A

formula:
~R
RC - Si-- R
OR
wherein R is a radical selected from the group consisting of alkyl, alkenyl, and aryl radicals and R
is a member of the group consisting of alkyl, alkenyl, aryl and alkoxy radicals.
According to the present invention there are provided two-part room temperature vulcanizing silicone compositions for taking dental impressions. Before curing, the compositions consist essentially~-of rom about 0.3 to about 0.7 parts by we.ight of a metallic salt of a monocarboxylic acid to 100 parts of a base compound cosisting essentially of:
(a) a fluid diorganopolysiloxane, (b) a filler therefor, (c) an organo silicon cross-linker having the general formula (Ro)3SiRl where R is a radical selected from the group consisting of alkylr alkenyl and aryl radicals and Rl is a member of the group consisting of alkyl, alkenyl, aryl and alkoxy radicals, the filler being selected from (i) an admixture of zlnc oxide, calcium carbonate and pumice, the zinc oxide and calci.um carbonate each being present in at least a sufficient amount to provide bulking and whitening, and said pumice being present in at least a sufficient amount to provide a putty-l:ike consistency, (ii) a low oil and sorption calcium carbonate, said fluid diorganopolysiloxane being selected from ~33~ 8SI-1384A

(iii) fluid diorganopolysiloxanes containing terminal silicon-bonded hydroxy groups and having a ViSC05 ity of from about 2,000 to 250,000 cps. at 25C, and (iv) diorganopolysiloxanes containing terminal silicon~bonded hydroxy groups and having a viscosity of from about 15,000 to about 35,000 cps. at 25C with the proviso that when said filler is of type (i) said fluid diorganopolysiloxane is of type (iii) and said base consists of by weight about 25 to about 35% (a), about 63 to about 75% (b) and about 0.05 to about 2% (c); and when said filler is of type (ii), said fluid dirogano-polysiloxane is of type (iv) and said base consists of by weight about 15 to about 25% (a), about 70 to about 85%
(b) and about 0.05 to about 2% (c) and additionally about 3 to about 8% of a mineral oil.
The preferred silicone fluid of type (iii) for the base compound is a silanol-stopped dimethylpolysiloxane (i.e., having terminal silicon bonded hydroxy groups) with a viscosity of from 3,000 to 200,000 cps. at 25C., and especially preferably a viscosity o~ ~rom 3,000 to 120,000 cps. at 25C. When polysiloxanes of 80,000 to 120,000 cps. are used, a low molecular weight silanol stopped polydimethylsiloxane containing 5 to 10 dimethyl disiloxane units and about 7% by weight of hydroxyl groups is employed in small but effective amounts to slow curing time. An especially effective amount of this low molecular weight silicone is from 0.05 to 2% by weight of the total base compound.
When a polyorganosiloxane of a lower viscosity value is employed, e.g., 2,500 to 3,500 cps. at 25 C., the base compound has high flow and is tacky to the touch prior to catalyzing and non-tacky after catalyzing.

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When a polyorganosiloxaneOf a relatively higher viscosity is used, e.g., 80,000 to 120,000 cps. at 25C., the base compound has low flow and is non-tacky to the touch prior to catalyzing.
The amount of catalyst employed with the base compound will vary depending upon the viscosity of the latter. When diorganopolysiloxanes having a viscosity of from 80,000 to 120,000 cps. at 25C. are employed in the base compound, a weight ratio of about 0.5 parts of catalyst to 100 parts of the base compound is preferred. With polysiloxanes of much lower viscosity, e.g., 2,500 to 3,500 cps., a lower amount of catalyst is used, such as about 0.4 parts of catalyst to lO0 parts of the base compound.
The filler of type (i) which is incorporated into the silicone fluid of type (iii) consists of an admixture to 3 to 15 parts, preferably 5 to 10, of zinc oxide, 5 to 25 parts, preferably lO to 20, of calcium carbonate and 45 to 65 parts, preferably 20 50 to 60, of pumice (A12O3-SiO2), based on lG0 parts by weight of the base. The various components of the filler work in combination. Calcium carbonate and zinc oxide are used as bulking as well as whitening agents. Pumice is used to provide a putty-like consistency to the composition. Pumice is also more easily wetted into the formulation and thus assures more consistency from batch to batch than other types of filler. Surprisingly, it has also been found that when pumice is used as a component of the filler, greater hardness is obtained with the composition within a relatively short time after addition of the catalyst and curing. All three components of the filler must be present to produce the advantageous ~33~

results achieved with this embodiment.
The preferable cross-linking agents are conventional organosilicon compounds used for this purpose having the general formula OR

~' RO - Sl - R

OR
wherein R is a radical selected from the group consîsting of alkyl, alkenyl and aryl radicals and Rl is a radical selected from the group consisting of alkyl, alkenyl, aryl and alkoxy radicals.
Some preferred cross-linking agents are phenyl triethoxy silane, vinyl triethoxysilane, n-propyl silicate and condensed ethyl orthosilicate.
Especially preferred is condensed ethyl orthosilicate.
Generally, from about 0.05 to about 2~ by weight of the ~otal base compound of the organo-silicon crosslinker is employed. Amounts less thanthis are generally insufficient to react with the organopolysiloxane to form the cured silicone rubber, ; and amounts larger than this tend to reduce the elasticity of the cured rubber.
Catalysts suitable for the present dental impression compositions are conventional and well known. Metal salts of monocarboxylic acids have been found to be effective. Various acid radicals and metal ions are suitable as components in the metal salts. Some preferred acid radicals are the linoleate, stearate, oleate, acetate, butyrate and octoate. Tin is especially preferred for the metal ion because of ~33~ 8SI-1384A

its low toxicity. Some preferred metal salts are tin oleate, tin butyrate and tin actoate. Especially preferred is tin oct~ate.
In a second embodiment there is provided a room temperature vulcanizing silicone dental impression composition which is non-sticky, or at most, slightly tacky and which cures to a smooth high durometer composition which does not have a "grainy", "gritty", or coarse feel. This embodiment, before curing, consists essentially of from about 0.~ to about 0.7 parts by weight of a tin salt of a monocarboxylic acid to 100 parts of a base compound consisting essentially of:
(a) from about 15 to about 25% by weight of a fluid diorganopolysiloxane of type (lv) containing terminal silicon-bonded hydroxy groups and having a viscosity of about 15,000 to about 25,000 cps at 25C ., tb) from about 70 to about 85~ by weight of a filler of type (ii) consisting essentially of low oil absorption calcium carbonate;
(c) from about 3 to about 8% by weight of mineral oil; and (d) from about 0.05 to about 2% by weight of an organo-silicon cross-linker having the general formula OR
I

RO - Si OR

wherein R is a radical selected from the group ~ 8SI-1384A

consisting of alkyl, alkenyl and aryl radicals and Rl is a member of the group consisting of alkyl, alkenyl, aryl and alkoxy groups, said composition having a non-sticky, or at most a slight "tacky"
feel before curing, and a smooth, non-grainy feel after curing. Some embodiments have a slight "gritty"
feel, which is not necessarily disadvantageous.
The diorganopolysiloxane of this embodiment ; is preferably a dimethylpolysiloxane having a 10 viscosity of from 15,000 to 35,000 cps. at 25 C.
and/ especially preferably, a viscosity of from about 20~000 to about 30,000 cps. at 25C.
Effective as cross-linking agents are the same organo-siliconcompounds mentioned above as suitable for the compositions of the first embodiment.
A relatively small amount of mineral oil is added to reduce or eliminate any stickiness in the base compound and facilitate its handling during the making of dental impressions. Suitable mineral oils will be the physio]ogically-acceptable bland fluids available from a number of commercial sources. One suitable mineral oil is the heavy white oil known as Kaydol TM Mineral Oil avaliable from Sonneborn Division of Witco Chemical Co.
For the catalyst, the same conventional metal salts of monocarboxylic acids mentioned above as suitable for the compositions of the first embodiment are also suitable for these compositions.
~ he filler is a low oil absorption calcium carbonate, which has been found to aid in providing a smooth, non-grainy appearance and feel to the cured silicone composition. Suitable grades of low oil -12~

~33~ 8SI-1384A

absorption calcium carbonates and their suppliers, are Camel Kote* (Harry T. Campbel and Sons Co., Towson, Maryland), Gamma Sperse* 255, Wingdale Whlte* and No.
10 White (Georgia Marble Company, Calcium Products Division, Tate, Georgia), Duramite* (Thompson, Weinman and Company, Montclair, New Jersey), and Micro White*
75 (Sylacauga Calcium Products Co., Sylacauga, Alabama).
Especially preferred is Camel Kote calcium carbonate.
The present invention is illustrated by the examples given below, which are not intended to limit the scope of the invention.

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~hese examples are illustrative of the preparation and properties of the compositions of the first embodiment referred to above. All proportions are on a percent by weight basis.
The base compound of Mixture A (Example 1) is prepared by thoroughly mixing 30.67 parts of silanol~stopped polydimethylsiloxane having a viscosity 20 of about 3,000 cps. at 25C., 12.26 parts of calcium carbonate reinforcing filler (Albacar*), 6.15 parts of zinc oxide filler (XX-78) 50.02 parts of pumice filler (Airfloat* 2599, commercially available from James H. Rhodes and Co., Long Island City, New York) and 0.90 parts of condensed ethyl orthosilicate. The blend is worked until a putty consistency is obtained.
This is high flow and tac~y to the touch. 5tamous octoate catalyst in the amount of about 0.4 parts per 100 parts of base compound is then worked into the putty to yield Mixture A, which cures at room temperature.

Mixture B (Example 2) is similarly prepared.
The base compound of B is ~irst obtained by mixing *TM

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thoroughly 30.43 parts of silanol-stopped polydimethyl-siloxane having a viscosity of about 120,000 cps. at 25 C., 12.16 parts of calcium carbonate filler ~Albacar3, 6.11 parts of zinc oxide filler (XX-78), 49.63 parts of pumice filler (Airfloate 2599), 0.89 parts of condensed ethyl orthosilicate, and to slow pot life, 0.78 parts of low molecular weight silanol-stopped polydimethyl-siloxane having about 7% by weight of hydroxy groups.
This composition is low flow and non-tacky to the touch.
10 To the resulting putty is added to about 0.6 parts of stannous octoate per 100 parts of base compound to obtain Mixture B, which cures at room temperature.
The properties of the compositions are surnmar-ized in Table 1:
TABLE 1: ROOM-TEMPERATURE VULCANIZING
DENTAL IMæ~ESSION COMPOSITIONS
Example (Mixture) l(A)2(B) Application Rate at 90 P.S.I.
in gm./min. 50-1500-30 20 Cure Time (sec.) 100-16090-150 Properties:
Specific Gravity 1.57-1.62 1.57-1.62 Linear Shrinkage (ADA Spec. 19) 0.2-0-3% 0.2-0.3 Shore A Hardness 10 Min. 60-7060-70 20 ~in. 70-8065-75 60 Min. 75-8570-80 ~ Elongation 50 59 Tear Die (lb./in.) 61 59 Color Gray Gray The foregoing properties indicate that ~3~ 8SI-1384A

extremely useful dental impression compositions have been provided.
The base compounds of example 1 and 2 exhibit good shelf aging properties for 6 months or more, based on accelerated aging at 50C.
Neither Mixtures A or B (Example 1 and 2) display any cure inhibition when used with custom trays made of polyvinylchloride, polyethylene, methyl methacrylate or epoxy materials.
The two examples given above represent compouns made of relatively high and low viscosity polyorganosiloxanes within the range of viscosities possible with the present compositions. Other blends of course, are possible, such as blends of Mixtures A and ~ to achieve properties between the two.

_ This example is illustrative of the preparation and properties of a composition of the second embodiment which is non-tacky and produces a very smooth cured vulcanizate. All proportions are on a percent by weight basis.
~- The base compound cosists of a mixture of 17 parts of silanol stopped polydimethylsiloxane having a viscosity in the range of 20,000 to 30/000 cps.
at 25C./ 5.2 parts of mineral oil (Wittco Chemical's Kaydol)/ 77 parts of low oil absorption calcium carbonate filler (Camel Kote), and 0.8 parts of 40~
condensed ethyl orthosilicate. The Camel Kote Filler has an estimated oil absorption value of about 10 30 lbs. oil/100 lbs. clacium carbonate, ASTM D-281-31.

The base compounds before catalyzation, has a putty-like consistency and exhibits good handling character-istics, being non-tacky and non-gritting to the touch.

This compound has a specific gravity of 1.85-1.90.
To initiate curing, starmous octoate catalyst is added and worked into the base compound in the ratio of 0.5 parts of catalyst per 100 parts of base compound. The resulting catalyzed dental impression composition has a work life of about 55 seconds at room temperature. After about 55 seconds has elapsed, the composition becomes difficult to handle or manipulate.

This example compares physical properties of toom-temperature vulcanizable dental impression compositions according to the present invention, using various low oil absorption calcium carbonates as the filler in the base compound. -;
Base compounds are prepared according to the procedure of Example 3 using a differen-t calcium carbonate filler in each case. The calcium carbonates employed, together with their corresponding oil absorption values, are summarized in Table 3.
TABLE 3: CALCIUM CARBONATE FILLERS
Oil Absorption Fillerlbs. oil/100 lbs. ~iller (ASTM D-281-31) Camel Kote* 10 Gamma Sperse* 255 10-12 No. 10 White7- 9 Wingdale* White 11-13 Duramite* 5- 6 Micro White* 75 10-12 The initial work life of the base compounds using each of the above fillers is measured after catalyzing a portion of each base compound according to *TM

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the procedure of Example 3. The Shore A hardnesses of the cataly~ed compounds are also measured at intervals of 5 and 10 minutes a~ter cat~lygation. These properties are summarized in Table 4.
To measure the stabillty of the various base compounds, the uncatalyzed portions of each of the compounds are shelf aged at 50C. for given time intervals (shown in Table 4), then catalyzed as above, and the work life and Shore A hardnesses are measured.
These properties are also summarized in Table 4.
TABLE 4: COMPARISON OF BASE COMPOUNDS USING ~ARIOUS
LOW OIL ABSORPTION CALCIUM CARBONATE FILLERS

Filler Work Life 5 min. 10 min.
(Secs.) Shore A Hardness Camel Kote*
Initial 55 66 63 After 245 days aging 55 76 73 Gamma Sperse* 255 Initial 50 70 80 After 233 days aging 55 60 72 No. 10 White Initial 60 70 73 After 217 days aging 60 37 50 Wingdale White*
Ini~ial 70 65 77 After 217 days aging 95 43 60 Duramite*
Initial 45 79 81 After 233 days aging 35 30 42 30 Microwhite* 75 Initial 60 68 75 After 217 days aging 60 53 68 *TM

~ ~ ,33~
8SI-138~A

The compositions employing Gamma Sperse*
~55 or Duramite* as the filler have a slightly gritty feel. The compositions which employ Wigndale White*
or Micro White* 75 are smooth but exhibit slight tack.
The composition containing No. 10 White exhibits a slight tack and has a slight gritty feel. The composition using Camel Kote* filler provides good handlingr i.e., non-tacky and non-gritty. It should be noted that these properties, i.e., "tack" and "gritty", are somewhat dependent on the subjective judgmenk of the individual who is doing the handling~
Although, all the above calcium carbonate fillers display acceptable properties, including handling, Camel Kote* calcium carbonate appears to give the best overall performance. This appears to be due in parts to the particle size distribution ~ of Camel Kote* calcium carbonate, which is unique in ; that it contains substantiall~ no particles above 30 microns in size. Thus while the oil absortion values of Camel Kote* and No. 10 White are about the same, Camel Kote* displays better handling character-istics, apparently because of the differences in particle size distribution between these two calcium carbonates.
It is to be understood that conventional additives may also be added to the present dental impression compositions, e.g., low-toxicity pigments and flavors.

O-ther compositions will suggest themselves 30 to those skilled in the art in view of the above- -detailed descriptions. All such obvious variations are within the full intended scope of the invention, which is de~ined in the appended claims.

*TM

Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A room temperature vulcanizing silicone dental impression composition which, before curing, consists essentially of from about 0.3 to about 0.7 parts by weight of a metallic salt of a monocarboxylic acid as a catalyst to 100 parts of a base composition consisting essentially of:
a) from about 15 to about 25% by weight of a fluid diorganopolysiloxane containing terminal silicon-bonded hydroxy groups and having a viscosity of from about 15,000 to about 35,000 centipoise at 25 C.;
b) from about 70 to about 85% by weight of a filler consisting essentially of low oil absorption calcium carbonate;
c) from about 3 to about 8% by weight of mineral oil; and d) from about 0.05 to about 2% by weight of an organolsilicon cross-linker having the general formula:

wherein R is a radical selected from the group consisting of alkyl, alkenyl and aryl radicals and R1 is a member of the group consisting of alkyl, alkenyl, aryl, and alkoxy groups, said composition having a non-sticky feel before curing, and a smooth, non-grainy appearance after curing.

2. A composition as defined in Claim 1 wherein the diorganopolysiloxane (a) is a dimethyl-polysiloxane.

3. A composition as defined in Claim 2 wherein the viscosity of said dimethyl polysiloxane is in the range of from 20,000 to 30,000 cps. at 25°C.

4. A composition as defined in Claim 1 wherein the organo-silicon cross-linker (d) is condensed ethyl orthosilicate.

5. A composition as defined in Claim 1 wherein the metallic salt is tin octoate.