patent for silicone in implants.....ingredients part 2

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Optionally, reinforcing and non-reinforcing inorganic fillers and

thixotropic additives, component (G), can also be included in the

composition. Component (G) comprises, for example, reinforcing and

non-reinforcing inorganic fillers and thixotropic additives such as fumed

silica, precipitated silica, finely powdered quartz, calcium carbonate,

talc, alumina, silicon nitride, aluminum nitride and titanium dioxide.

Hydrophobized fumed silica is especially preferred because it prevents

crepe hardening in the stored polyorganosiloxane composition prior to its

curing. Blends of hydrophobized and hydrophilic silica also provide a

safeguard against excessive solids settling in the stored material prior

to curing. Some elastomers, especially elastomers that must be

electrically conductive, are filled with finely powdered metal such as

copper, silver, gold, or platinum particles. Such products are described

in U.S. Pat. Nos. 4,770,641; 5,037,312; and 5,074,799, the complete

disclosures of which are incorporated herein by reference.

Specific thixotropes that can be employed in conjunction with fumed silica

and other fillers include the commercial products sold under the trade

names, KEVLAR ULTPATHIXâ„¢, TROYTHIXâ„¢ XYZ and THIXCINâ„¢. KEVLAR

ULTPATHIX filler is a fibrous form of poly(p-phenyleneterephthalamide)

manufactured and sold by DUPONT as a thixotrope. TROYTHIX XYZ and THIXCIN

fillers are both triglycerides derived from glycerol and castor oil fatty

acids. TROYTHIX is a trademark of TROY CORPORATION and THIXCIN is a

trademark of BAKER CASTOR OIL CO.

Component (G) can also comprise pigments and dyes used to color the gels

and elastomers.

In many applications, the polyorganosiloxane compositions are cured in situ

in an enclosure. For example, curing can be effected in an envelope made

of a polyurethane-polyester copolymer, or of a polyurethane-polyether

copolymer, or of polydimethylsiloxane-polydiphenylsiloxane copolymer.

Descriptions of envelope materials have been published in Rubber Chemistry

and Technology, 56(1983) 523-556; J. Biomaterials Applications, 3(1988)

228-259; J. Chromatography, 349(1985) 347-356 and J. Biomedical Materials

Research, 24(1990) 1,585-1,598. The complete disclosures of these

references are incorporated herein by reference.

In practice, the polymers used as barrier materials are often fabricated as

thin films and laminates in contact with disposable backings made of

paper, polyethylene or polypropylene. The films can be cast from solvents

or from the melt. Release agents are used to facilitate separation of the

film from the backing. Additionally, the film can contain antioxidants and

other specialty ingredients to protect the film from oxidation, heat,

light and biodegradation.

These ingredients and/or their thermal decomposition products can inhibit

the curing of the polyorganosiloxane composition. Along with the

above-mentioned release agents, these ingredients can also inhibit

adhesive bonding of the cured polyorganosiloxane to the envelope material.

However, the adhesion promoting crosslinker © permits excellent adhesive

bonding of the cured elastomer to envelope and substrate materials,

irrespective of the method of film fabrication, release agents and/or

special protective additives.

The self-adherent polyorganosiloxane gel and elastomer compositions of the

instant invention are made by mixing the components (A)-(G) described

hereinabove in proportions hereinafter defined, placing the resultant

mixture in an appropriate container or on an appropriate surface, and

curing the mixture with heat at a temperature from about 25° C. up

to about 200° C., and preferably, between about 70° C. and

about 150° C., for a period up to about 4 hours. As is well known,

longer times are associated with conditions such as lower temperatures,

lower catalyst levels, inhibited catalysts, and lower crosslinker

concentrations.

If all the ingredients are mixed together in a one part formulation, the

order of mixing is important to prevent premature curing of the

composition. Thus, the catalyst, even when inhibited, is typically the

last ingredient added to a one part formulation. Alternatively, the

ingredients can be combined selectively in a two part formulation. Mixing

of the two parts occurs just prior to curing. The two parts are mixed in a

gravimetric ratio that corresponds to the Si--H/vinyl stoichiometry

yielding the desired cure kinetics and gel or elastomer properties. The

essential criterion used in formulating the separate parts is the

segregation of the hydrosilylation catalyst and the Si--H crosslinker. The

person having ordinary skill in the art can determine appropriate mixing

methods without undue experimentation.

The amount of components (A)-(G) depends on the intended use of the gel or

elastomer and can be readily determined by the person having ordinary

skill in the art. For example, if the gel is to be used in an external

breast prosthesis, then the following composition can be used. The amount

of component (A) should be at least about 10 weight percent and maximally

about 95 weight percent of the total formulation. The preferred amount is

between about 15 and about 35 weight percent.

The amount of component ( is selected Such that the stoichiometric ratio

of Si--H groups to unsaturated groups in component (A) is between about

0.3 and about 10 and, preferably, between about 0.8 and about 2. Thereby,

the gravimetric content of component ( in the formulation typically can

be between about 0.1 and about 50 weight percent, preferably between about

0.4 and about 10 weight percent.

The quantity of component © can be between about 0.1 and about 5.0 Weight

percent, preferably between about 0.01 and about 1.0 weight percent, and

most preferably, is between about 0.05 and about 0.5 weight percent.

The catalytic amount of component (D) depends on the desired work time and

curing rate. A broad range between about 0.1 and about 100 ppm Pt based on

the total weight of the formulation is normally effective. The preferred

range is between about 2.5 and about 15 ppm Pt. The person having ordinary

skill in the art can determine optimal catalyst use.

Component (E) can be between about 30 and about 90 weight percent and is,

preferably, between about 70 and about 85 weight percent.

The effective level of component (F) is also determined by the desired work

time and processing conditions. Moreover, as will be shown by examples

below, the temporary catalyst inhibitors have different intrinsic

inhibitive tendencies. For example, considerably less diethyl maleate or

methylvinylcyclosiloxane is required on a stoichiometric basis relative to

platinum than octylsilane to achieve the same degree inhibition. Effective

levels of fillers and thixotropes (component G) can account for between

about 0.01 and about 5 weight percent of the total formulation. Amounts

between about 1.0 and about 2.5 weight percent are preferred. Pigments

such as, for example, flesh-toned lighter and darker shades, can be

optionally included in the formulation in appropriate amounts determined

readily by the person having ordinary skill in the art.

Components A and D typically constitute the bulk of the mass of a gel

formulation. The contents of the other ingredients can be expressed

relative to the combined weights of these two components.

The proportions of the ingredients can also be expressed as stoichiometric

ratios. Thus, the ratio of hydrosilane, Si--H, equivalents from both the

network ( and adhesion promoting © crosslinkers to unsaturated

equivalents such as vinyl equivalents can be, for example, between about

0.5 and about 7.5, and preferably, between about 0.8 and about 6. The

ratio of Si--H equivalents from the network crosslinker ( to those from

adhesion promoting crosslinker © can be, for example, between about 0.1

and about 3, and preferably, between about 0.2 and about 2.

As a formulation parameter to control the processing and properties of

curable polysiloxane compositions, it is well known to use the variation

of the stoichiometric ratio between equivalents of Si--H in the network

crosslinker B and equivalents of the unsaturated group such as a vinyl

group in component A. Gel firmness follows an approximately parabolic

profile with increasing Si--H/vinyl ratio, whereas work time and gel time

decrease logarithmically.

Tackiness and adhesion of the cured composition are generally decreased

with increasing Si--H/vinyl ratio. However, Si--H/vinyl ratios higher than

those used with conventional network crosslinkers can be employed in the

formulation described herein without loss of desirable processing and

properties. In fact, work time is desirable extended, tack and adhesion

are improved and firmness remains controllable at the higher Si--H/vinyl

ratios.

The SiH3 -containing adhesion promoting crosslinker is preferably

included in the siloxane composition during curing so that the cured

elastomer or gel is self-adherent to its substrate. The compositions of

the invention are also useful as primers. For instance, the SiH3

-containing compound may also be used in a separate step as a primer to

improve the bonding between the surface of the elastomer or gel and a

substrate.

A primer, as defined in J. Shields, ADHESIVES HANDBOOK, Butterworths,

London, Second Edition (1976) p 341, is a surface coating applied

beforehand to improve bonding of the surface to an adhesive or any

overlayer. U.S. Pat. No. 4,401,500 and Japanese Patents 84/220,347,

84/220,348, 84/220,349 disclose primer compositions for improving the

adhesion of siloxane compositions to metals, glass and plastics. However,

none of these compositions comprises the use of an SiH3 -containing

compound.

When used as a primer, the SiH3 -containing compound may be applied to

surfaces with a brush, or as a spray or by any of the methods known in the

art for this treatment. The SiH3 -containing compound may be used

neat, or dissolved in a readily vaporized solvent which will not

deteriorate the properties of the surfaces and their adhesion to the

elastomer or gel. Heptane, hexane, toluene, xylene, ethyl acetate, butyl

propionate, ethanol isopropanol, amyl alcohol, 2-ethylhexanol,

trichloroethylene, chloroform, methylene chloride, trichlorofluoromethane

and supercritical carbon dioxide and their miscible combinations are all

suitable solvents for the SiH3 -containing primers, but the list is

not limited to these alone. The quantity of solvent is not narrowly

critical. One skilled in the art can determine the concentration of

SiH3 -containing compound which affords the optimum viscosity for the

method of application and minimum solvent evaporation time.

Priming permits the use of those SiH3 -containing compounds which are

too volatile for inclusion in the self-adherent formulations. CH3

SiH3, C4 H9 SiH3, Si2 H6 and H3

SiOSiH3 are examples of such compounds. Substrates can be treated

with solutions of these primers, or exposed directly to atmospheres

containing these volatile compounds. Priming can be performed in-line

during a continuous manufacturing process, or as a separate earlier unit

operation. In both cases, sufficient time must be allowed for evaporation

of the solvent at the processing temperature.

The following illustrative and comparative examples describe the instant

invention in more detail. However, they are not intended to limit the

scope of the specification and the claims.

EXAMPLES

Materials

Terminal vinylsiloxane fluids were used in the experiments as component

(A). One fluid had a viscosity of about 2,000-2,500 centistokes and a

vinyl content of about 0.24±0.02 weight percent; the other a viscosity

of about 60,000-70,000 centistokes and a vinyl content of about

0.07±0.01 weight percent. These fluids are referred to in the examples

as vinyl fluid (2,000 cstk) and vinyl fluid (60,000 cstk), respectively. A

trimethylsiloxy terminated dimethylsiloxane oil of viscosity 350

centistokes was the plasticizer/rheology modifier of component (E). The

network crosslinkers (, MD15 D'5.5 M, MD20 D'3.2 M,

and MD43.2 D'6.8 M, (M.dbd.(CH3)3 SiO1/2 ;

D.dbd.(CH3)2 SiO; D'.dbd.CH3 SiHO) were used in the

experiments. Octylsilane, C8 H17 SiH3, phenylsilane,

C6 H5 SiH3, and octadecylsilane, C18 H37

SiH3, were the adhesion promoting crosslinkers ©.

Polyurethane-polyester films and external breast prosthesis bags used in

the tests are commercial materials sold, for example by Atochem and

& Nephew, Ltd. These films are often supplied with a polyethylene or paper

backing that was removed just prior to the experiments described

hereinbelow. The backing is typically used to control static electricity

and facilitate handling. The film surface that was contacted with the

curing polysiloxane formulation was the surface not covered by the

polyethylene or paper backing.

A platinum catalyst (D) referred to as PCAT I was prepared according to the

method described by Karstedt in U.S. Pat. No. 3,775,452, the complete

disclosure of which is incorporated herein by reference. The complex of

1,3-divinyltetramethyldisiloxane so prepared was dissolved in silicone

oil, 500 cstk, to obtain a stock solution containing about 2.5-3 wt % Pt.

Another platinum catalyst referred to as PCAT II was made from

methylvinylsiloxane cyclic tetramer and cyclic trimer and

hexachloroplatinic acid dissolved in isopropanol, as described in British

Patent Nos. 1,228,376 and 1,228,377, the complete disclosures of which are

incorporated herein by reference. Pt content of this catalyst was 3.2 wt

%. A 1 cc syringe was used to dispense the small quantities of catalyst

required for some experiments.

Gel Testing

Gel firmness Was measured with a penetrometer fitted with a 1/4 size, 2.5

gram grease cone, 7.0 gram shaft according to ASTM D1403-86, "Standard

Test Method for Cone Penetration of Lubricating Grease Using One-Quarter

and One-Half Scale Cone Equipment." Measurements are shown in 1/10

millimeter, a unit Standard in the art. Lower values indicate firmer gels.

Values between about 70 and about 100 correspond to a life-like feel in

the gel and are most desirable for external mammary prostheses. However,

values outside of this range are acceptable for gels that are required to

be harder or softer for other purposes.

Cure time and gel time were measured in the following ways. In the first

way, gel firmness was measured as a function of time following the

addition of catalyst to a one part formulation, or of time following

mixing of both parts of a two part formulation. The time required to

attain a stable reading is the cure time. In the second way, a Bholin

Stress Rheometer was used to measure and record the loss (that is,

viscosity, G"), and storage (that is, elasticity, G'), moduli, dynamic

modulus, G*, and phase angle, d, between the stress and strain of

the curing gel/elastomer. The time for gelation is approximately the time

at which the loss and storage moduli intersect. References related to

these rheological measurements include S. K. Venkataram et al. Polymer

Preprints, 29(1988) pgs. 571-572; C. W. Mackosko, et al. Macromolecules,

9(1976) 199; and E. E. Holly, et al., J. Non-Newtonian Fluid Mechanics,

27(1988) 17-26. The cure time is the point at which the dynamic modulus

attains a constant or near constant value. The ratio of the loss modulus

to the storage modulus, G"/G', is equal the tangent of the phase angle, or

tan d. Tan d measures the damping ability of the cured or

curing polysiloxane composition. Tan d values less than about 0.1,

and preferably, between about 0.01 and about 0.08, are desirable for the

gels contained in external breast prostheses.

Pull strength was measured with the Instron Model 1123 using samples cut

from a cured gel sandwich. A gel sandwich was prepared between two sheets

of polyurethane-polyester film in a stainless steel mold that had internal

dimensions 11.3 cm× 7.5 cm×3 mm. The sandwich was sealed in

the mold and cured at 125° C. for 45 minutes. The length of the

polyurethane-polyester film extended beyond the gel boundary in the mold

to facilitate attachment of the sample to the Instron during pull strength

measurement. For this, the cooled, cured sandwich was cut longitudinally

into three equal slices of dimensions 11.3 cm×2.5 cm×3 mm.

Thus, triplicate measurements were performed for each sandwich made. The

sample was supported on a small lab jack elevated to the height of the

bottom clamp on the Instron. The excess polyurethane-polyester film was

appropriately attached at the top to the tensile load cell, and at the

bottom, to the stationary clamp of the Instron. A pulling force was

applied to the top film such that the load cell moved upwards at 5 in/min.

The force was recorded in grams. Values greater than or equal to 100 grams

are desirable. Additionally, it is desired that the adhesive strength of

the gel to the polyurethane-polyester film be greater than its cohesive

strength.

A qualitative, manual pull strength test was also done. In this test, a gel

sample was cured in contact with a strip of polyurethane-polyester film.

After the sample was removed from the oven, the strip was pulled away from

the cured gel to determine the locus of adhesion failure. If the film

peeled away cleanly from the gel surface with no adherent gel, then

failure occurred at the gel-film interface, and adhesive failure occurred.

Adhesive failure is undesirable. If the gel broke and tore during the pull

test and gel remained firmly adherent to the film, cohesive failure

occurred. Cohesive failure is desirable because it is indicative of strong

bonding at the gel-film interface.

Example 1

This example illustrates that compounds © that have the primary silane

functionality, SiH3, obtain good adhesion of the cured gel to

polyurethane-polyester films. The films used were U01 and U073 from

Atochem and a sample supplied by & Nephew Ltd.

A mixture with the following composition was made from the indicated raw

materials

______________________________________

RAW MATERIAL PARTS BY WEIGHT

______________________________________

Vinylsiloxane Fluid (2,000 cstk)

14.67

Vinylsiloxane Fluid (60,000 cstk)

7.33

Silicone Oil (350 cstk)

78.00

______________________________________

Forty gram samples of this mixture were used in each experiment. The

quantities of network crosslinker, (MD43.2 D'6.8 M), adhesion

promoting crosslinker (octylsilane, phenylsilane or octadecylsilane) are

shown in Table 1. Percentages are reported relative to the combined

weights of silicone oil and vinylsiloxane fluids (40 gm). 0.01 gm PCAT I

(equivalent to 6.3 ppm Pt) was used in the experiments with octylsilane.

Twice that amount was used with phenylsilane and octadecylsilane.

In each experiment, the ingredients were combined in a 300 ml waxed

paper-cup that was capable of withstanding temperatures up to 175°

C. and stirred mechanically at about 1,500 rpm for about 45 seconds. The

sample was then deaerated under vacuum for 5 minutes. A sharp razor blade

was used to trim the paper cup down to the level of its contents, and

sections about 8 cm wide by 10 cm long of polyurethane-polyester film were

placed gently on the surface of the liquid. Samples were then cured in an

oven at 125° C. for 45 minutes. A duplicate sample without

overlaying film was cured for the measurement of gel penetration. U073

film, 75 micron thick, was used in all the experiments shown in Table 1.

Excellent adhesion of gel to film was also observed in tests done with 50

micron thick polyurethane-polyester film from & Nephew. The results

of the qualitative pull tests illustrate the necessity for SiH3

functionalized compounds to obtain desirable adhesion of cured gel to the

film. The data also show that desirable adhesion and gel firmness are

realized over broad ranges of concentration for network crosslinker,

MD43.2 D'6.8 M, and adhesion promoting crosslinkers, n--C8

H17 SiH3, C6 H5 SiH3, and C18 H37

SiH3. The stoichiometric ratio of Si--H equivalents from the network

crosslinker to those from the SiH3 -containing compound spanned

0.2-1.4, whereas the stoichiometric ratio of Si--H equivalents to vinyl

equivalents spanned 0.8-5.5.

TABLE 1

______________________________________

EFFECT OF n-C8 H17 SiH3, C6 H5 SiH3 AND

C18 H37 SiH3

ON FILM-GEL ADHESION

______________________________________

MD43.2 D'6.8 M

n-C8 H17 SiH3

RESULTS

gm wt % gm wt % PENET. ADHES.

______________________________________

0.279 0.698 -- -- nm adh.

0.279 0.698 0.04 0.100 nm coh.

0.290 0.725 -- -- 109 adh.

0.290 0.725 0.047 0.118 91 coh.

0.200 0.500 0.06 0.150 86 coh.

0.328 0.82 0.06 0.150 65 coh.

0.681 1.703 0.09 0.225 83 coh.

0.690 1.725 0.084 0.210 80 coh.

0.726 1.815 0.091 0.228 82 coh.

______________________________________

MD43.2 D'6.8 M

n-C6 H5 SiH3

RESULTS

gm wt % gm wt % PENET. ADHES.

______________________________________

0.280 0.700 0.014 0.035 59 coh.

0.280 0.700 0.021 0.053 57 coh.

0.280 0.700 0.036 0.090 56 coh.

______________________________________

MD43.2 D'6.8 M

n-C18 H37 SiH3

RESULTS

gm wt % gm wt % PENET. ADHES.

______________________________________

0.280 0.700 0.079 0.198 63 coh.

______________________________________

PENET. = PENETRATION, 1/10 Mm, ASTM D1403-86

ADHES. = ADHESION

adh. = adhesive failure at gel/film interface in manual pull test

coh. = cohesive gel breakage in manual pull test

nm = not measured

Example 2

This example illustrates the use of two additional network crosslinkers

with octylsilane to make self-adherent polysiloxane compositions useful as

gels in external breast prostheses. Both U073 and the & Nephew, S & N,

polyurethane-polyester films were tested for gel-film adhesion. A 4 cm

wide by 8 cm long section of each film was applied to the surface of each

sample prior to cure. Otherwise, the sample preparation and test procedure

were those described in Example 1. Forty grams of the blend of

vinylsiloxane fluids and silicone oil and a platinum concentration of 6.3

ppm were used in each experiment. Table 2 sets forth the quantities of

network crosslinkers and octylsilane, as well as experimental results.

The results show that the levels of network and adhesion promoting

crosslinkers afforded gels that have excellent gel-film bonding.

TABLE 2

______________________________________

IMPROVED GEL-FILM ADHESION WITH OCTYLSILANE

AND MD15 D'5.5 M OR MD20 D'3.2 M

______________________________________

CROSSLINKER

MD15 D'5.5 M

n-C8 H17 SiH3

ADHESION

gm wt % gm wt % S & N U073 PENET.

______________________________________

0.155 0.388 -- -- adh. adh. 82

0.155 0.388 0.040 0.100 coh. coh. 72

______________________________________

CROSSLINKER

MD15 D'3.2 M

n-C8 H17 SiH3

ADHESION

gm wt % gm wt % S & N U073 PENET.

______________________________________

0.285 0.713 -- -- adh. adh. 100

0.285 0.713 0.042 0.105 coh. coh. 91

0.285 0.713 0.144 0.360 coh. coh. 84

______________________________________

Penet. units are in 1/10 mm.

Example 3

This example illustrates the preparation of external breast prostheses

using a curable polysiloxane composition comprising the network

crosslinker (MD43.2 D'6.8 M), and an adhesion promoting

crosslinker (n--C8 H17 SiH3).

The two part formulation had the following composition. All values are in

grams.

______________________________________

COMPONENT PART A PART B

______________________________________

Vinylsiloxane Fluid, 2,000 cstk

34.33 34.33

Vinylsiloxane Fluid, 60,000 cstk

17.17 17.17

Silicone Oil (350 cstk)

198.5 198.5

MD43.2 D'6.8 M

3.588 --

n-C8 H17 SiH3

0.458 --

PCAT I -- 0.014

Flesh-tone pigment 0.096 --

______________________________________

Each part was blended separately with a Cowles Dissolver at 1,500 rpm for

15 minutes. 220 gm of each part was blended together with a mechanical

stirrer at 1,500 rpm for 5 minutes and deaerated under vacuum for 10

minutes. With the aid of a 100 ml syringe, an external prosthesis bag

fabricated from U073 film was filled with 136 gm of the pink, flesh toned

deaerated liquid. Another bag fabricated with U073 film on the back side

and & Nephew film on the front was similarly filled with 252 gm of

the pink, flesh toned deaerated liquid. Both bags were heat sealed and

later clamped into metal molds that correspond to various breast sizes and

shapes. Cure occurred in an oven at 125° C. for 90 minutes. The

unused reaction mixture was set aside at room temperature for estimation

of the working time of the formulation.

The cured prostheses had a life-like feel, resilience and responsiveness.

Gel to film adhesion was checked on the front and back of each prosthesis

by slitting the devices with a razor blade to isolate a section about 2 cm

wide by 4 cm long for manual pull testing. Cohesive gel breakage was

observed in each case. Moreover, the gel was not readily scraped away from

the film of the test section. These observations confirm that

n-octylsilane imparts excellent adhesion to the gel formulation not only

in small test samples as in Example 1, but also in external breast

prostheses.

The unused reaction mixture was still fluid and able to be poured after 16

hours at room temperature. Thus the working time of the formulation was in

excess of 16 hours.

Example 4

This comparative control illustrates the effect of omitting the adhesion

promoting crossliner from the breast prosthesis formulation.

The two part formulations employed were similar to that shown in Example 3,

except that the n--C8 H17 SiH3 was omitted from PART A. In

one experiment, the weight of the network crosslinker, MD43.2

D'6.8 M, in PART A was kept unchanged at 3.588 grams, while in

another the weight was increased to 8.886 grams to make the stoichiometric

ratio of SiH groups to vinyl groups equal to that of Example 3. The

composition of PART B was unaltered.

Prostheses were prepared at room temperature and cured at 125° C. as

described in Example 3. Crosslinking, indicated by an observable increase

in the viscosity of the blended formulation and the difficulty of filling

the bags manually by syringe, was occurring at ambient temperature even as

the prosthesis bags were being filled. The pot-life was <30 minutes for

both experiments.

The cured prostheses showed no gel to film adhesion. The prosthesis made

with the lower level of network crosslinker was soft and limp. The

prosthesis made with the higher level was hard and rubbery. Both lacked

the life-like feel, resilience and responsiveness described in Example 3

for the prosthesis containing n--C8 H17 SiH3. Although it

is possible to obtain this feel by control of the SiH/Vinyl stoichiometry,

desirable gel to film adhesion will still not be realized without the

incorporation of adhesion promoter. :

Example 5

This example illustrates the increase in gel time which results from the

use of n-octylsilane.

Forty grams of the blend of vinylsiloxanes and silicone oil defined in

Example 1 were used together with 0,288 grams of the network crosslinker

(MD43.2 D'6.8 M), and the quantities of the other formulation

components indicated in Table 3. The platinum concentration was 6.3 ppm.

Samples were prepared as described in Example 1, except that film strips

were not overlaid. A small amount of each reaction mixture was applied to

the cone and plate stage of the Bholin Stress Rheometer, which was

controlled at 25° C.

The data show that gel time is increased nearly 60-fold by the addition of

0.1 wt. % n--C8 H17 SiH3 to the formulation. Thus,

n--C8 H17 SiH3 is a potent temporary catalyst inhibitor.

TABLE 3

______________________________________

EFFECT OF OCTYLSILANE ON GEL TIME

SAMPLE CURE TEMP, °C.

GEL TIME,sec

______________________________________

No Additive 25 1,521.2

(25.35 min.)

0.1 wt % n-C8 H17 SiH3

25 91,020.1

(1,517 min.)

______________________________________

Example 6

This example illustrates further the temporary inhibitive effect of

octylsilane on the crosslinking of siloxane compositions.

Samples were formulated with the blend of vinylsiloxane and siloxane oil

described in Example 1 and quantities of network crosslinker (MD43.2

D'6.8 M) and adhesion promoter (n--C8 H17 SiH3), to

give the SiH/Vinyl stoichiometries shown in Table 4. In the calculation of

SiH/Vinyl stoichiometry, each mole of n--C8 H17 SiH3

contributes 3 SiH equivalents and each mole of network crosslinker

contributes 6.8 SiH equivalents. Platinum concentration was held constant

at 4-4.6 ppm in all of the experiments of this example. PCAT I was the

catalyst used.

Cure of the gels occurred on the stage of the Bholin Stress Rheometer using

a heating rate of 2.5° C./min from 25° C. up to 150°

C. Dynamic modulus, (G*), followed a sigmoidal profile with increasing

temperature. The rapid increase of G* began at an initiation temperature

corresponding to the onset of cure in the gel sample. This initiation

temperature is determined by the specific nature and amount of the

temporary catalyst inhibitor. Higher initiation temperatures are

associated with a greater extent of temporary inhibition. Lower initiation

temperatures reflect increasing ease of siloxane cure.

Gelation was initiated at 42° C., 40° C. 37° C. and

34° C. as SiH/Vinyl stoichiometry was increased from 0,834 to 2.12

in the four comparative examples of Table 4. These samples contained the

network crosslinker (MD43.2 D'6.8 M) and no n--C8 H17

SiH3. The data show that gel cure is facilitated by an increase in

SiH/Vinyl stoichiometry when the network crosslinker is the only source of

the SiH groups. The logarithm of the initiation temperature correlates

linearly and negatively with SiH/Vinyl stoichiometry.

Table 4 shows that in the presence of n--C8 H17 SiH3,

initiation temperature increased as SiH/Vinyl stoichiometry was increased.

In fact, the logarithm of the initiation temperature correlates linearly

and positively with the SiH/Vinyl stoichiometry. This means that addition

of n--C8 H17 SiH3 to the siloxane formulation caused a

delay in its curing. However, the samples containing n--C8 H17

SiH3 exhibited excellent gel-film adhesion with U073 film and

acceptable gel firmness, penetration and dynamic moduli.

TABLE 4

______________________________________

EFFECT OF n-C8 H17 SiH3 ON THE INITIATION

TEMPERATURE FOR GEL CURE

SAMPLE DESCRIPTION

SIH/VINYL INIT. TEMP., °C.

______________________________________

COMPARATIVE

EXAMPLES

0.70 wt % MD43.2 D'6.8 M

0.834 42

0.85 wt % MD43.2 D'6.8 M

1.112 40

1.17 wt % MD43.2 D'6.8 M

1.526 37

1.62 wt % MD43.2 D'6.8 M

2.120 34

EXAMPLES CONTAIN-

ING 0.70 wt %

MD43.2 D'6.8 M AND

0.02 wt % n-C8 H17 SiH3

1.112 53

0.05 wt % n-C8 H17 SiH3

1.526 62

0.10 wt % n-C8 H17 SiH3

2.120 88

______________________________________

Example 7

This Example illustrates the comparative effects of temporary catalyst

inhibitors on an addition cure formulation suitable for silicone coatings

on thermoplastic materials, for example, polyurethane-polyester or

styrene-olefin-butadiene block copolymers. The overall siloxane

composition of the formulation is shown in Table 5. The inhibitors used

and their concentrations are summarized in Table 6. PCAT I was the

catalyst used.

For each experiment summarized in Table 6, the molar quantities of

inhibitor and platinum were added with manual mixing into 80 gm aliquots

of the siloxane composition. Samples were cured isothermally on the stage

of the Bholin Stress Rheometer maintained at 25° C. Gel time was

measured at the intersection of the storage and loss moduli as explained

hereinabove and illustrated in Example 4. Longer gel times indicate a

higher extent of catalyst inhibition.

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