Storage stable curable liquid silicone composition

Abstract

A process for preparing a curable liquid silicone composition includes: (a) combining the following components to form a catalyst / inhibitor masterbatch: (i) platinum hydrosilylation catalyst; (ii) tetrahydrocarbylthiuram disulfide that has a flowing point of 50 °C or higher; and (iii) aromatic silicone carrier fluid; and (b) combining the catalyst / inhibitor masterbatch with a vinyl-functional silicone and a silyl hydride functional silicone to form a curable composition.

Description

[0001] STORAGE STABLE CURABLE LIQUID SILICONE COMPOSITION

[0002] FIELD

[0003] The present invention relates to curable silicone compositions and methods for preparing them.

[0004] INTRODUCTION

[0005] Curable liquid silicone compositions are useful as secondary insulators for electric motors because they can provide a desired level of electrical insulation and thermal protection. Curable liquid silicone compositions for such applications typically contain a silicone that has a vinyl (Vi) functionality and a silicone crosslinker that contains multiple silyl hydride (SiH) functionalities. The curable liquid silicone composition also typically comprises a hydrosilylation catalyst. The Vi and SiH groups undergo hydrosilylation to cure the composition. Hydrosilylation occurs more rapidly at elevated temperatures, but it can also occur at lower temperatures and that can result in storage instability for the curable liquid silicone compositions.

[0006] An approach to increasing the storage stability of curable liquid silicone compositions is to add an inhibitor to deter hydrosilylation curing at lower temperatures. US4260726, for instance, describes adding organic compounds containing a [=N-C(-S-)2-] sub-unit to a curable liquid silicone composition in order to reduce the viscosity build of the curable composition when aged at 80 °C (indicative of increasing storage stability) while still achieving rapid cure reactivity at 175 °C. However, the method of US4260726 still results in viscosity build of the composition at temperatures of 80 °C. It is desirable to identify a way to obtain storage stability of curable liquid silicone compositions that results in less viscosity build during storage than is obtainable by the method disclosed in US4260726 and yet still achieves rapid onset of curing at temperatures above 120°C.

[0007] SUMMARY

[0008] The present invention is partly a result of discovering a particular challenge with achieving storage stability of curable liquid silicone compositions that comprise an inhibitor to deter hydrosilylation curing at lower temperatures when the inhibitor has a flowing point of 50 °C or higher. When an inhibitor has a flowing point of 50 °C or higher, the inhibitor takes longer to dissociate into a composition and hydrosilylation reaction can occur causing viscosity build as it dissociates.

[0009] The present invention provides a way to prepare a curable liquid silicone composition using an inhibitor with a flowing point of 50 °C or higher, where the curable liquid silicone composition demonstrates enhanced storage stability as evidenced by less viscosity build during storage than is obtained by simply adding an inhibitor to a composition of catalyst and reactive silicone composition, such as is described in US426076. Testing of liquid silicone compositions prepared according to the method disclosed in US426076 have demonstrated a viscosity increase of 40% or more when stored at 80 °C for 150 hours. Liquid silicone compositions prepared according to the process of the present invention experience a viscosity build of 20% or less, even 10% or less, when stored at 80 °C for 150 hours even when using an inhibitor with a flowing point of 50 °C or higher. Moreover, liquid silicone compositions prepared according to the process of the present invention demonstrate rapid curing as indicated by a DSC Exotherm Sharpness of 25 °C or less when using a temperature ramping rate of 20 °C / minute from 25 °C to 300 °C with an onset temperature of exotherm (start of curing) that is above 120 °C. Yet moreover, the process of the present invention can contain less than 5 wt%, even less than 2 wt% , even less than one wt% and can be completely free of organic solvent so that the liquid silicone compositions prepared according to the present invention have less than 5 wt%, less than 2 wt% , less than one wt%, or even are free of organic solvent.

[0010] Surprisingly, the present invention is a result of discovering that by first adding the hydrosilylation catalyst and inhibitor together to form a masterbatch and then adding that masterbatch to a curable liquid silicone the storage stability of resulting curable silicone composition is much better than for a curable silicone composition formed by adding inhibitor to a composition already containing the curable liquid silicone and catalyst. Without being bound by theory, it is possible that the inhibitor is able to dissociate and complex with the catalyst when mixed together to form the masterbatch without the catalyst initiating a hydrosilylation reaction as the inhibitor dissociates. Complexing the catalyst with the inhibitor prevents the catalyst from catalyzing curing of the curable liquid silicone composition while still dissociating sufficiently to demonstrate rapid cure when heated to temperatures above 120 °C. In contrast, when catalyst and inhibitor having a flowing point of 50 °C or higher are added separately to a composition that already contains both the curable liquid silicone then curing and viscosity build can occur before the solid inhibitor can dissociate and complex with the catalyst.

[0011] In a first aspect, the present invention is a process for preparing a curable liquid silicone composition, the process comprising: (a) combining the following components to form a catalyst / inhibitor masterbatch: (i) platinum hydrosilylation catalyst; (ii) a tetrahydrocarbylthiuram disulfide that has a flowing point of 50 °C or higher; and (iii) aromatic silicone carrier fluid; and (b) combining the catalyst / inhibitor masterbatch with a vinyl-functional silicone and a silyl hydride functional silicone to form a curable composition.

[0012] The present invention is useful for forming curable silicone composition that has enhanced storage stability.

[0013] DETAILED DESCRIPTION

[0014] Test methods refer to the most recent test method as of the priority date of this document when a date is not indicated with the test method number. References to test methods contain both a reference to the testing society and the test method number. The following test method abbreviations and identifiers apply herein: ASTM refers to ASTM International methods; END refers to European Norm; DIN refers to Deutsches Institut fur Normung; ISO refers to International Organization for Standards; and UL refers to Underwriters Laboratory.

[0015] Products identified by their tradename refer to the compositions available under those tradenames on the priority date of this document.

[0016] “Multiple” means two or more. “And / or” means “and, or as an alternative”. All ranges include endpoints unless otherwise indicated.

[0017] “Silicone” refers to a polysiloxane, which is a molecule that comprises multiple siloxane units. Identification of siloxane units often utilize abbreviations M, D, T and Q. M-type siloxane units refer to units having the chemical formula: RasSiOi / 2. D-type siloxane units refer to units having the chemical formula: Ra2SiO2 / 2- T-type siloxane units refer to units having the chemical formula: RaSiOs / 2- Q-type siloxane units refer to units having the chemical formula: SiO.4 / 2. In these general formulae, each Rais independently in each occurrence selected from a hydrogen, hydrocarbyl group (substituted or nonsubstituted), hydroxyl, alkoxyl, or essentially any other group bound to the silicon atom.

[0018] The O’s refer to oxygen atoms bound the silicon that are bound to a silicon atom of another siloxane unit. The subscript is a multiple of 14 to reflect that the oxygen is bound to this silicon atom and another silicon atom of another siloxane unit also having a multiple of V2 in the denominator - both siloxane units reflect ownership of 14 of the same oxygen atom. The number in the oxygen subscript reflects how many oxygens are bound to the specified silicon atom that are also bound to another siloxane unit silicon atom. Typically, there are subscripts associated with the siloxane units themselves to indicate the relative amounts of the siloxane unit in the molecule. If the subscripts associated with siloxane units are greater than one, then the subscript refers to the average number of those siloxane units in the molecule. If the subscript associated with siloxane units is less than one, then the subscript refers to the average molar ratio of the siloxane unit associated with the subscript relative to total moles of all siloxane units in the molecule. Subscripts of one are typically left unstated so if a siloxane unit does not include a subscript it is understood to have a subscript of one. A chemical formula for a silicone typically lists the siloxane units in blocks, but that does not necessarily imply block polymerization (that is, that the siloxane units exist in the molecule as blocks) but rather are presented in block for convenience to indicate how much of each siloxane unit is present total in the polymer.

[0019] A “resinous polysiloxane”, or “resin”, contains 30 mole-percent (mol%) or more and can contain 50 mol% or more, 70 mol% or more, 90 mol% or more , even 100 mol% of Q- type, T-type or a sum of Q-type and T-type siloxane units. In contrast, a “non-resinous” silicone, which is often referred to simply as a “polymer”, “polymeric”, or “linear” silicone, siloxane or polysiloxane, contains less than 30 mol% of a combination Q-type and T-type siloxane units and often contains only M-type and D-type siloxane units.

[0020] “Silyl hydride” functionality refers to having a hydrogen atom bonded directly to a silicon atom to form an SiH group.

[0021] “DSC Exotherm Sharpness” refers to the temperature range defined by the onset of exotherm to peak exotherm temperature. Or, put another way, DSC Exotherm Sharpness is the value of (Peak Exotherm Temperature) - (Onset Temperature of Exotherm). DSC Exotherm Sharpness is a measure of how rapidly a composition cures once curing begins with a shorter value corresponding to a faster curing.

[0022] “Solid” refers to a state of matter that does not perceptively flow to the unaided eye. “Flowing point” refers to a melting point for crystalline materials and glass transition temperature for amorphous materials. If a material has both a melting point and a glass transition temperature then the flowing point is the lower of the two. Basically, the flowing point refers to the temperature at which a solid (non- flowing) material transitions to a flowable state. Determine flowing point for a material by differential scanning calorimetry (DSC) using ASTM method D3418.

[0023] The present invention is a process for preparing a curable liquid silicone composition. “Curable” means that composition has components that can react with one another in an additive way to form a crosslinked material. “Liquid” means the composition is flowable at 25 °C and 101 kiloPascals pressure. As a “silicone composition”, the composition comprises at least one silicone component.

[0024] The process comprises two steps: (a) combining the following components to form a catalyst / inhibitor masterbatch: (i) platinum hydrosilylation catalyst; (ii) tetrahydrocarbylthiuram disulfide; and (iii) aromatic silicone carrier fluid; and (b) combining the catalyst / inhibitor masterbatch with a vinyl-functional silicone and a silyl hydride functional silicone to form a curable composition.

[0025] The catalyst / inhibitor masterbatch comprises: (i) platinum hydrosilylation catalyst; (ii) a tetrahydrocarbylthiuram disulfide that has a flowing point of 50 degrees Celsius (°C) or higher; and (iii) aromatic silicone carrier fluid. Prepare the catalyst / inhibitor master batch by combining those three components. Other components may or may not also be combined with those three components in forming the catalyst / inhibitor masterbatch. In the broadest scope of the invention, the components of the masterbatch can be combined in any order and in any way. Particularly for smaller scale preparation, it can be useful to first disperse individual components in aromatic silicone carrier fluid prior to mixing them together to facilitate uniform mixing of the components to form the masterbatch. For example, the inhibitor can be mixed with aromatic silicone carrier fluid and separately catalyst can be mixed with aromatic silicone carrier fluid and then those two mixtures combined to form a masterbatch. As a result, the catalyst / inhibitor masterbatch can comprise or consist of the result of combining (i) platinum hydrosilylation catalyst; (ii) tetrahydrocarbylthiuram disulfide that has a flowing point of 50 °C or higher; and (iii) aromatic silicone carrier fluid.

[0026] The platinum hydrosilylation catalyst can be any one or any combination of more than one catalyst containing platinum that is useful in catalyzing hydrosilylation reactions. Platinum hydrosilylation catalysts include compounds and complexes such as platinum (0)- l,3-divinyl-I,I,3,3-tetramethyldisiloxane (Karstedt’s catalyst), HLPtCT,, di- p. -carbonyl di- .7t.-cyclopentadienyldinickel, platinum-carbonyl complexes, platinum- divinyltetramethyldisiloxane complexes, platinum cyclovinylmethylsiloxane complexes, platinum acetylacetonate (acac), platinum black, platinum compounds such as chloroplatinic acid, chloroplatinic acid hexahydrate, a reaction product of chloroplatinic acid and a monohydric alcohol, platinum bis(ethylacetoacetate), platinum bis(acetylacetonate), platinum dichloride, and complexes of the platinum compounds with olefins or low molecular weight polyorganosiloxanes or platinum compounds microencapsulated in a matrix or core-shell type structure.

[0027] In the broadest scope of the present invention, the tetrahydrocarbylthiuram disulfide is any one or any combination of more than one component selected from a group consisting of tetraalkylthiuram disulfides and tetraarylthiuram disulfides that has a flowing point of 50 °C or higher, preferably 60 °C or higher, and can be 65 °C or higher, while at the same time is typically 200 °C or lower, preferably 180 °C or lower, more preferably 170 °C or lower and can be 160 °C or lower. Preferably, the tetrahydrocarbylthiuram disulfide is any one or any combination of more than one component selected from a group consisting of tetrabenzylthiuram disulfide (flowing point of 124 °C per SigmaAldrich), tetramethylthiuram disulfide (flowing point of 156-158 °C per SigmaAldrich), tetraethylthiuram disulfide (flowing point of 69-71 °C per SigmaAldrich), tetra(iso- propyl)thiuram disulfide (flowing point of 115-117 °C per SigmaAldrich) and tetra(iso- butyl)thiuram disulfide (flowing point of 73.5-74.5 °C per ChemBK). In contrast, tetra(n- butyl)thiuram disulfide (flowing point of 33 °C per Fisher Scientific) is outside the scope of the inhibitor for the present invention.

[0028] The concentration of tetrahydrocarbylthiuram disulfide is desirably such that the molar ratio of tetrahydrocarbylthiuram disulfide to platinum in the hydrosilylation catalyst is in a range of 0.9 to 3.

[0029] The composition comprises an aromatic silicone carrier fluid. Desirably, aromatic silicone fluids are non-resinous silicones that have pendant aromatic groups. Examples of two suitable aromatic silicone fluids for use as aromatic silicone carrier fluid include those having the chemical composition of (I) and (II), respectively:

[0030] [(CH3)2ViSiOl / 2]2[(CH3)PhSiO2 / 2]m(I)

[0031] [(CH3)2(OH)SiOl / 2]2[(CH3)PhSiO2 / 2]n (ID where “Vi” refers to a vinyl group; “Ph” refers to a phenyl group; subscript m has a value of 5 or more, and can be 10 or more, 20 or more, 25 or more, 30 or more, 40 or more, 50 or more, 75 or more, 100 or more, 150 or more, 200 or more, 250 or more, 300 or more, 350 or more, 400 or more, even 450 or more while at the same time is typically 500 or less, and can be 475 or less, 425 or less, 375 or less, 325 or less, 275 or less, 225 or less, 175 or less, 125 or less, 75 or less, 50 or less, 40 or less, even 30 or less; and subscript n has a value of 2 or more, and can be 5 or more, 10 or more, 20 or more, 25 or more, 30 or more, 40 or more, 50 or more, 75 or more, 100 or more, 150 or more, 200 or more, 250 or more, 300 or more, 350 or more, 400 or more, even 450 or more while at the same time is typically 500 or less, and can be 475 or less, 425 or less, 375 or less, 325 or less, 275 or less, 225 or less, 175 or less, 125 or less, 75 or less, 50 or less, 40 or less, 30 or less, 20 or less, 10 or less, or even 8 or less.

[0032] The vinyl-functional silicone and the silyl hydride functional silicone can be different silicone molecules or they can be the same molecule having both functionalities. Desirably, both the vinyl-functional silicone and the silyl hydride functional silicone are silicone resins, or are the same silicone resin with both functionalities. Desirably, the vinylfunctional silicone and the silyl hydride functional silicone are the same silicone resin having both functionalities. When the vinyl-functional silicone and the silyl hydride functional silicone are the same silicone, the composition can still further comprise an additional vinyl-functional silicone and / or silyl hydride functional silicone. Alternatively, when the vinyl-functional silicone and the silyl hydride functional silicone are the same silicone, the composition can be free of additional vinyl-functional silicone and / or silyl hydride functional silicone.

[0033] Desirably, the vinyl-functional silicone and the silyl hydride functional silicone are the same silicone resin that has the chemical structure (III):

[0034] (PhSiO3 / 2)a(ViMeSiO2 / 2)b(HMeSiO2 / 2)c[(Me)3SiOi / 2]d (III) where “Ph” refers to a phenyl group, “Vi” refers to a vinyl group, “Me” refers to a methyl group subscripts a, b, c and d refer to molar ratios of the associated siloxane unit relative to moles of all siloxane units in the molecule. Subscript a has a value of 0.3 or more, and can be 0.4 or more, 0.43 or more and at the same time is typically 0.7 or less and can be 0.5 or less, even 0.45 or less. Subscript b has a value of 0.05 or more, and can have a value of 0. 10 or more, 0.12 or more, even 0.14 or more while at the same time typically has a value of 0.2 or less, and can be 0.15 or less. Subscript c typically has a value of 0.05 or more and can be 0.13 or more, 0.14 or more, 0.15 or more, even 0.16 or more while at the same time is typically 0.2 or less and can be 0.18 or less, even 0.17 or less. Subscript d generally has a value of 0. 15 or more, and can be 0.20 or more, even 0.25 or more while at the same time is typically 0.35 or less, and can be 0.30 or less, even 0.26 or less.

[0035] The molar ratio of SiH functionality to vinyl functionality in the curable liquid silicone composition is desirably 0.8 or more, and can be 1.0 or more, 1.1 or more, 1.2 or more, 1.3 or more, 1.4 or more, even 1.5 or more while at the same time is desirably 2 or less.

[0036] Desirably, the concentration of the components combined in the process of the present invention are such that the curable composition comprises a combination of platinum hydrosilylation catalyst at a concentration sufficient to provide platinum at a concentration of 0.2 to 50 weight-parts per million weight parts of curable liquid silicone composition, tetrahydrocarbylthiuram disulfide at a concentration sufficient to provide a molar ratio of tetrahydrocarbylthiuram disulfide to platinum from the hydrosilylation catalyst that is in a range of 0.9 to 3, aromatic silicone carrier fluid concentration that is in a range of 0.5 to 10 wt% based on weight of curable liquid silicone composition, and a molar ratio of SiH functionality to vinyl functionality that is in a range of 0.8 to 2.

[0037] The curable liquid silicone composition of the present invention demonstrates a rapid cure reactivity as indicated by a DSC Exotherm Sharpness of 25 °C or less when using a temperature ramping rate of 20 “ / minute from 25 °C to 300 °C. Curing can initiate at temperatures of 200 °C or higher, 175 °C or higher, 150 °C or higher, or even 120 °C or higher. Curing typically has a peak temperature in the DSC while determine DSC Exotherm Sharpness that is less than 275 °C. The curable liquid silicone composition demonstrates stability from curing at temperatures of 80 °C even after aging for 150 hours or more as evidenced by a viscosity increase of less than less than 20% after aging.

[0038] EXAMPLES

[0039] Prepare the examples using the materials listed in Table 1.

[0040] Table 1

[0041] Examples (Exs) 1-4

[0042] Prepare Exs 1 -4 by first preparing a master batch of Pt Catalyst and inhibitor in aromatic silicone carrier fluid. Add silicone resin to the masterbatch and blend together to produce a curable liquid silicone composition.

[0043] Ex l

[0044] Step (1). Place into a 20 gram (g) volume dental mixer cup 0.330 g Pt Catalyst (containing 25.4 wt% Pt metal) and 9.715 g of Aromatic Silicone Carrier Fluid 1 and then mix the components together with a planetary mixer at 3000 revolutions per minute (RPM) for five minutes.

[0045] Step (2). Into a 40 g volume dental mixer cup add 0.125 g Inhibitor 1 and 19.92 g of Aromatic Silicone Carrier Fluid 1 and mix the components together with a planetary mixer at 3000 RPM for five minutes.

[0046] Step (3). In a small cup, add 10.775 g of the mixture of Inhibitor 1 and Aromatic Silicone Carrier Fluid 1 from Step (2) and 23.12 g of Aromatic Silicone Carrier Fluid 1 and then blend together using a planetary mixer at 3000 RPM for five minutes. Step (4). To the mixture from Step (3) add 6.142 g of the mixture from Step (1) and blend with a planetary mixer at 3000 RPM for two minutes to form a catalyst / inhibitor masterbatch.

[0047] Step (5). Into a small cup, add 50.001 g of Difunctional Silicone Resin and 1.01 g of the catalyst / inhibitor masterbatch from Step (4) and then blend together with a planetary mixer at 3000 RPM for two minutes to produce a curable liquid silicone composition (Example 1).

[0048] Ex 2

[0049] Step (1). Place into a 40 g volume dental mixer cup 0.923 g Pt Catalyst (containing 25.4 wt% Pt metal) and 19.09 g of Aromatic Silicone Carrier Fluid 1 and then mix the components together with a planetary mixer at 3000 revolutions per minute (RPM) for five minutes.

[0050] Step (2). Into a 40 g volume dental mixer cup add 0.377 g Inhibitor 2 and 19.628 g of Aromatic Silicone Carrier Fluid 1 and mix the components together with a planetary mixer at 3000 RPM for five minutes.

[0051] Step (3). In a small cup, add 4.046 g of the mixture of Inhibitor 2 and Aromatic Silicone Carrier Fluid 1 from Step (2) and 31.986 g of Aromatic Silicone Carrier Fluid 1 and then blend together using a planetary mixer at 3000 RPM for five minutes.

[0052] Step (4). To the mixture from Step (3) add 4.028 g of the mixture from Step (1) and blend with a planetary mixer at 3000 RPM for two minutes to form a catalyst / inhibitor masterbatch.

[0053] Step (5). Into a small cup, add 50.157 g of Difunctional Silicone Resin and 1.012 g of the catalyst / inhibitor masterbatch from Step (4) and then blend together with a planetary mixer at 3000 RPM for two minutes to produce a curable liquid silicone composition (Example 2).

[0054] Ex 3

[0055] Step (1). Place into a 20 g volume dental mixer cup 0.304 g Pt Catalyst (containing 25.4 wt% Pt metal) and 9.70g of Aromatic Silicone Carrier Fluid 2 and then mix the components together with a planetary mixer at 3000 revolutions per minute (RPM) for five minutes. Step (2). Into a 40 g volume dental mixer cup add 0. 111 g Inhibitor 1 and 19.90 g of Aromatic Silicone Carrier Fluid 2 and mix the components together with a planetary mixer at 3000 RPM for five minutes.

[0056] Step (3). In a small cup, add 10.97 g of the mixture of Inhibitor 1 and Aromatic Silicone Carrier Fluid 2 from Step (2) and 23.31 g of Aromatic Silicone Carrier Fluid 2 and then blend together using a planetary mixer at 3000 RPM for five minutes.

[0057] Step (4). To the mixture from Step (3) add 6.137 g of the mixture from Step (1) and blend with a planetary mixer at 3000 RPM for two minutes to form a catalyst / inhibitor masterbatch.

[0058] Step (5). Into a small cup, add 5.029 g of Difunctional Silicone Resin and 1.109 g of the catalyst / inhibitor masterbatch from Step (4) and then blend together with a planetary mixer at 3000 RPM for two minutes to produce a curable liquid silicone composition (Example 3).

[0059] Ex 4

[0060] Step (1). Place into a 20 g volume dental mixer cup 0.214 g Pt Catalyst (containing 25.4 wt% Pt metal) and 10.948g of Aromatic Silicone Carrier Fluid 1 and then mix the components together with a planetary mixer at 3000 revolutions per minute (RPM) for five minutes.

[0061] Step (2). Into a 40 g volume dental mixer cup add 0.110 g Inhibitor 6 and 21.418 g of Aromatic Silicone Carrier Fluid 1 and mix the components together with a planetary mixer at 3000 RPM for five minutes.

[0062] Step (3). In a small cup, add 17.798 g of the mixture of Inhibitor 6 and Aromatic Silicone Carrier Fluid 1 from Step (2) and 213.486 g of Aromatic Silicone Carrier Fluid 1 and then blend together using a planetary mixer at 3000 RPM for five minutes.

[0063] Step (4). To the mixture from Step (3) add 9.069 g of the mixture from Step (1) and blend with a planetary mixer at 3000 RPM for two minutes to form a catalyst / inhibitor masterbatch.

[0064] Step (5). Into a 100 g volume dental mixer cup, add 60.001 g of Difunctional Silicone Resin and 1.202 g of the catalyst / inhibitor masterbatch from Step (4) and then blend together with a planetary mixer at 3000 RPM for two minutes to produce a curable liquid silicone composition (Example 4). Comparative Examples (Comp Exs) A and B

[0065] Prepare Comp Exs A and B by combining a mixture of Pt Catalyst and Difunctional Silicone Resin with a mixture of Inhibitor (two different inhibitors, one for each Comp Ex) and Difunctional Silicone Resin to produce a curable liquid silicone composition. This procedure contrasts Exs 1-4 by being free of aromatic silicone carrier fluid and by blending the silicone resin with catalyst apart from inhibitor rather than forming a masterbatch of catalyst and inhibitor prior to exposing the catalyst or inhibitor to the silicone resin.

[0066] Comp Ex A

[0067] Step (1). Into a small cup add 0.024 g of Pt Catalyst and 49.337 g of Difunctional Silicone Resin and then blend them using a planetary mixer at 3000 RPM for five minutes.

[0068] Step (2). Into a small cup add 0.0021 g of Inhibitor 1 and 46.606 g of Difunctional Silicone Resin and blend using a planetary mixer at 300 RPM for five minutes.

[0069] Step (3). Into a small cup add 10.883 g of Difunctional Silicone Resin and add to that 38.495 g of the mixture from Step (2) and then add 10.66 g of the mixture from Step (1). Blend the components together using a planetary mixer at 3000 RPM for 5 minutes to form a curable liquid silicone composition (Comp Ex A).

[0070] Comp Ex B

[0071] Step (1). Into a small cup add 0.024 g of Pt Catalyst and 49.337 g of Difunctional Silicone Resin and then blend them using a planetary mixer at 3000 RPM for five minutes.

[0072] Step (2). Into a small cup add 0.0093 g of Inhibitor 2 and 50.155 g of Difunctional Silicone Resin and blend using a planetary mixer at 300 RPM for five minutes.

[0073] Step (3). Into a small cut add 37.35 g of Difunctional Silicone Resin and to that add 12.003 g of the mixture from Step (2), followed by adding 10.73 g of the mixture from Step (1). Blend the components together using a planetary mixer at 3000 RPM for 5 minutes to form a curable liquid silicone composition (Comp Ex B).

[0074] Comp Exs C and D

[0075] Prepare Comp Exs C and D by combining Difunctional Silicone Resin to a mixture of Pt Catalyst in aromatic silicone carrier fluid. Then add inhibitor (two different inhibitors, one for each Comp Ex) and mix to form a curable liquid silicone composition. This procedure contrasts Exs 1-4 by blending the silicone resin with catalyst apart from inhibitor, and silicone resin with inhibitor, rather than forming a masterbatch of catalyst and inhibitor prior to exposing the catalyst or inhibitor to the silicone resin.

[0076] Corny Ex C

[0077] Step (1). Into a small cup add 3.14 g of Pt Catalyst and 76.84 g of Aromatic Silicone Carrier Fluid 1 and then blend with a planetary mixer at 3000 RPM for five minutes to make a 1% solution of catalyst in aromatic silicon carrier fluid.

[0078] Step (2). Into a small cup, add 49.885 g of Difunctional Silicone Resin and 0.114 g of the mixture from Step (1) and then blend with a planetary mixer at 3000 RPM for five minutes.

[0079] Step (3). Add 0.00171 g of Inhibitor 1 to the mixture from Step (2) and blend with a planetary mixer at 3000 RPM for 5 minutes to make a curable liquid silicone composition (Comp Ex C).

[0080] Comp Ex D

[0081] Step (1). Into a small cup add 3.14 g of Pt Catalyst and 76.84 g of Aromatic Silicone Carrier Fluid 1 and then blend with a planetary mixer at 3000 RPM for five minutes to make a 1% solution of catalyst in aromatic silicone carrier fluid.

[0082] Step (2). Into a small cup, add 49.852 g of Difunctional Silicone Resin and 0.120 g of the mixture from Step (1) and then blend with a planetary mixer at 3000 RPM for five minutes.

[0083] Step (3). Add 0.002 g of Inhibitor 2 to the mixture from Step (2) and blend with a planetary mixer at 3000 RPM for 5 minutes to make a curable liquid silicone composition (Comp Ex D).

[0084] Comp Exs E and F

[0085] Prepare Comp Exs E and F by combining Difunctional Silicone Resin to a mixture of Pt Catalyst in organic carrier fluid. Then add inhibitor (two different inhibitors, one for each Comp Ex) and mix to form a curable liquid silicone composition. This procedure contrasts Exs 1-4 by blending the silicone resin with catalyst apart from inhibitor, and silicone resin with inhibitor, rather than forming a masterbatch of catalyst and inhibitor prior to exposing the catalyst or inhibitor to the silicone resin. Comp Ex E

[0086] Step (1). Into a small cup add 1.57 g of Pt Catalyst and 38.44 g of Carrier Fluid 3 to make a one % solution of catalyst in toluene. Mix using a planetary mixer at 3000 RPM for five minutes.

[0087] Step (2). Into a small cup add 49.748 g of Difunctional Silicone Resin and 0.111 g of the mixture from Step (1). Mix using a planetary mixer at 3000 RPM for five minutes.

[0088] Step (3). Into a small cup, add 0.175 g of Inhibitor 1 and 17.36 g of Carrier Fluid 3 and blend with a planetary mixer at 3000 RPM for 2 minutes to form a mixture. Add 0.149 g of this mixture to the mixture from Step (2) and mix using a planetary mixer at 3000 RPM for five minutes to make a curable liquid silicone composition (Comp Ex E).

[0089] Comp Ex F

[0090] Step (1). Into a small cup add 1.57 g of Pt Catalyst and 38.44 g of Carrier Fluid 3 to make a one % solution of catalyst in toluene. Mix using a planetary mixer at 3000 RPM for five minutes.

[0091] Step (2). Into a small cup add 49.748 g of Difunctional Silicone Resin and 0.108 g of the mixture from Step (1). Mix using a planetary mixer at 3000 RPM for five minutes.

[0092] Step (3). Into a small cup, add 0.216 g of Inhibitor 2 and 17.44 g of Carrier Fluid 3 and blend with a planetary mixer at 3000 RPM for 2 minutes to form a mixture. Add 0.151 g of this mixture to the mixture from Step (2) and mix using a planetary mixer at 3000 RPM for five minutes to make a curable liquid silicone composition (Comp Ex F).

[0093] Comp Exs G and H

[0094] Prepare Comp Exs G and H using a mixing procedure similar to that for Exs 1-3, except use different inhibitors than used in Exs 1-4. Comp Exs G and H illustrate the need to use tetrahydrocarbylthiuram disulfide inhibitors in the process of the present invention to achieve the desired rapid curing of the resulting curable liquid silicone composition.

[0095] Comp Ex G

[0096] Step (1). Place into a 40 gram (g) volume dental mixer cup 0.959 g Pt Catalyst (containing 25.4 wt% Pt metal) and 19.193 g of Aromatic Silicone Carrier Fluid 1 and then mix the components together with a planetary mixer at 3000 revolutions per minute (RPM) for three minutes. Step (2). Into a 40 g volume dental mixer cup add 0.44g Inhibitor 3 and 9.829 g of Aromatic Silicone Carrier Fluid 1 and mix the components together with a planetary mixer at 3000 RPM for five minutes.

[0097] Step (3). In a small cup, add 5.94 g of the mixture of Inhibitor 3 and Aromatic Silicone Carrier Fluid 1 from Step (2) and 3. 137 g of Aromatic Silicone Carrier Fluid 1 and then blend together using a planetary mixer at 3000 RPM for five minutes.

[0098] Step (4). To the mixture from Step (3) add 0.979 g of the mixture from Step (1) and blend with a planetary mixer at 3000 RPM for two minutes to form a catalyst / inhibitor masterbatch.

[0099] Step (5). Into a small cup, add 5.003 g of Difunctional Silicone Resin and 0.114 g of the catalyst / inhibitor masterbatch from Step (4) and then blend together with a planetary mixer at 3000 RPM for two minutes to produce a curable liquid silicone composition (Comparative G).

[0100] Comp Ex H

[0101] Step (1). Place into a 40 gram (g) volume dental mixer cup 0.959 g Pt Catalyst (containing 25.4 wt% Pt metal) and 19.193 g of Aromatic Silicone Carrier Fluid 1 and then mix the components together with a planetary mixer at 3000 revolutions per minute (RPM) for five minutes.

[0102] Step (2). Into a 20 g volume dental mixer cup add 0.94 g of mixture from Step (1), 0.516 g Inhibitor 4, 0.522 g of Inhibitor 5, 8.034 g of Aromatic Silicone Carrier Fluid 1 and mix the components together with a planetary mixer at 3000 RPM for three minutes to form a catalyst / inhibitor masterbatch.

[0103] Step (3). Into a small cup, add 5.016 g of Difunctional Silicone Resin and 0.098 g of the catalyst / inhibitor masterbatch from Step (2) and then blend together with a planetary mixer at 3000 RPM for two minutes to produce a curable liquid silicone composition (Comparative H).

[0104] Comp Ex I and J

[0105] Prepare Comp Ex I using a mixing procedure similar to that for Exs 1-4, except use tetra(n-butyl)thiuram disulfide as the inhibitor Prepare Comp Ex J using a mixing procedure similar to that for Comp Exs C and D. Comp Ex I and J illustrate that when the inhibitor is a liquid at a temperature below 50 °C then storage stability is adequate whether the curable composition is made using a masterbatch procedure or not - illustrating that the storage stability problem is a unique challenge when using an inhibitor that has a flowing point of 50 °C or higher in reactive siloxane compositions.

[0106] Comp Ex I

[0107] Step (1). Place into a 40 g volume dental mixer cup 0.923 g Pt Catalyst (containing 25.4 wt% Pt metal) and 19.09g of Aromatic Silicone Carrier Fluid 1 and then mix the components together with a planetary mixer at 3000 revolutions per minute (RPM) for five minutes.

[0108] Step (2). Into a 20 g volume dental mixer cup add 0.261 g Inhibitor 7 and 9.762g of Aromatic Silicone Carrier Fluid 1 and mix the components together with a planetary mixer at 3000 RPM for five minutes.

[0109] Step (3). In a small cup, add 4.00 g of the mixture of Inhibitor 7 and Aromatic Silicone Carrier Fluid Ifrom Step (2) and 32.006 g of Aromatic Silicone Carrier Fluid 1 and then blend together using a planetary mixer at 3000 RPM for five minutes.

[0110] Step (4). To the mixture from Step (3) add 4.054 g of the mixture from Step (1) and blend with a planetary mixer at 3000 RPM for two minutes to form a catalyst / inhibitor masterbatch.

[0111] Step (5). Into a small cup, add 50.095 g of Difunctional Silicone Resin and 1.011 g of the catalyst / inhibitor masterbatch from Step (4) and then blend together with a planetary mixer at 3000 RPM for two minutes to produce a curable liquid silicone composition (Comparative Example I).

[0112] Comp Ex . /

[0113] Step (1). Into a small cup add 3.14 g of Pt Catalyst and 76.848 g of Aromatic Silicone Carrier Fluid 1 and then blend with a planetary mixer at 3000 RPM for five minutes to make a 1% solution of catalyst in aromatic silicon carrier fluid.

[0114] Step (2). Into a small cup, add 49.887 g of Difunctional Silicone Resin and 0.110 g of the mixture from Step (1) and then blend with a planetary mixer at 3000 RPM for five minutes.

[0115] Step (3). Add 0.002 g of Inhibitor 7 to the mixture from Step (2) and blend with a planetary mixer at 3000 RPM for 5 minutes to make a curable liquid silicone composition (Comp Ex J). Sample Characterization

[0116] Characterize each of the samples of curable liquid silicone compositions for storage stability and for rapid cure. Test methods are below and results are in Table 2.

[0117] Storage Stability

[0118] Characterize storage stability by measuring the extent of viscosity increase as a sample ages. The objective is to achieve less than a 20% viscosity increase after aging 150 hours at 80 °C. Measure viscosity of samples with a Brookfield DV3T cone / plate (CP40) viscometer with Haake K20 / DC3 water circulating bath maintaining a sample temperature of 25 °C. Measure an initial viscosity immediately after preparing the sample. Then age the sample and measure the viscosity of the aged sample. Age the samples in an oven at 80 °C for 150 hours unless otherwise noted.

[0119] Rapid Cure Testing

[0120] Evaluate the speed of cure of the samples using differential scanning calorimetry (DSC). Use a TA instruments Q2000 DSC with a liquid nitrogen cooling system. Use 10 milligram samples in a perforated DSC pan. Collect a DSC scan using a sampling interval of one second per point. First equilibrate the sample at -100 °C and then ramp the sample to 300 °C at a rate of 20 °C per minute. Note the temperature of initial exotherm (Onset Temperature of Exotherm) and the peak temperature achieved (Peak Exotherm Temperature). If the difference Onset Temperature and Peak Exotherm Temperature, that is, the DSC Exotherm Sharpness, is less than 25 °C, then the sample is considered to undergo “rapid cure”. If the temperature difference (DSC Exotherm Sharpness) is greater than 25 °C then the sample is not considered to undergo a “rapid cure”. Put another way, if curing continues for 25 minutes or longer (the temperature ramp is one degree per minute) then the curing is not considered “rapid cure”.

[0121] Results

[0122] Comparing results from Exs 1-4 against results from Comp Exs A-F reveal that premixing the inhibitor and catalyst to form a masterbatch prior to exposing the reactive silicone(s) to the catalyst results in a significantly more stable reactive composition as compared to procedures where the catalyst is combined with reactive resin(s) without prior mixing with inhibitor as evidenced by experiencing less than 20% viscosity increase after aging at 80 °C for 150 hours. Without being bound by theory, it is believed that the catalyst and inhibitor form a stabilizing complex when premixed together and that complex precludes the catalyst from driving a reaction of the reactive silicone(s). However, when heated to above 200 °C the complex appears to free the catalyst to trigger a rapid curing. The benefit of the process steps of the present invention are not evident with any inhibitor, rather only tetrahydrocarbylthiuram disulfide inhibitors. Comparing DSC results of Exs 1-4 to those of Comp Exs G and H reveal use of a different type of inhibitor fails to result in rapid curing of the resulting curable liquid silicone composition.

[0123] Table 2

[0124] *Aged for 162 hours. **Aged 144 hours.

Claims

CLAIMSWhat is claimed is:

1. A process for preparing a curable liquid silicone composition comprising:(a) combining the following components to form a catalyst / inhibitor masterbatch:(i) platinum hydrosilylation catalyst;(ii) tetrahydrocarbylthiuram disulfide that has a flowing point of 50 degrees Celsius and higher; and(iii) aromatic silicone carrier fluid; and(b) combining the catalyst / inhibitor masterbatch with a vinyl-functional silicone and a silyl hydride functional silicone to form a curable composition.

2. The process of claim 1 , wherein the tetrahydrocarbylthiuram disulfide is selected from a group consisting of tetrabenzylthiuram disulfide, tetramethylthiuram disulfide, tetraethylthiuram disulfide, tetra(iso-propyl)thiuram disulfide, and tetra(iso-butyl)thiuram disulfide.

3. The process of any one previous claim, wherein the platinum hydrosilylation catalyst is Karstedt’ s catalyst.

4. The process of any one previous claim, wherein the molar ratio of tetrahydrocarbylthiuram disulfide to platinum in the hydrosilylation catalyst is in a range of 0.9 to 3.

5. The process of any one previous claim, wherein the aromatic silicone carrier fluid is any one or combination of more than one material having chemical composition (I) or (II):[(CH3)2ViSiOi / 2]2[(CH3)PhSiO2 / 2]m(I)[(CH3)2(OH)SiOi / 2]2[(CH3)PhSiO2 / 2]n (II) where “Vi” refers to a vinyl group; “Ph” refers to a phenyl group; subscript m has a value of 5 or more while at the same time is 500 or less; and subscript n has a value of 2 or more while at the same time is 500 or less.

6. The process of any one previous claim, wherein the vinyl-functional silicone and a silyl hydride functional silicone are silicone resins.

7. The process of any one previous claim, where in the vinyl-functional silicone and a silyl hydride functional silicone are the same silicone that contains both vinyl functionality and silyl hydride functionality.

8. The process of any one previous claim, wherein the vinyl-functional silicone has the chemical formula: (PhSiO3 / 2)a(ViMeSiO2 / 2)b(HMeSiO2 / 2)c[(Me)3SiOi / 2]d; wherein Ph refers to a phenyl group, Vi refers to a vinyl group, Me refers to a methyl group, subscripts a, b, c and d refer to molar ratios of the associated siloxane unit relative to moles of all siloxane units in the molecule and subscript a is in a range of 0.3 to 0.7, subscript b is in a range of 0.05 to 0.2, subscript c is in a range of 0.05 to 0.2, and subscript d is in a range of 0.15 to 0.35.

9. The process of any one previous claim, wherein the concentration of platinum hydrosilylation catalyst is in a range of 0.2 to 50 weight-parts per million weight parts of curable liquid silicone composition, the concentration of tetrahydrocarbylthiuram disulfide is sufficient to achieve a molar ratio of tetrahydrocarbylthiuram disulfide to platinum from the hydrosilylation catalyst that is in a range of 0.9 to 3, the concentration of aromatic silicone carrier fluid is up to 10 weight percent of the curable liquid silicone composition, and the molar ratio of SiH to vinyl functionality is in a range of 0.8 to 2.

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