WO2001049774A2

WO2001049774A2 - Organosiloxane compositions

Info

Publication number: WO2001049774A2
Application number: PCT/US2001/000221
Authority: WO
Inventors: François DEBUYL; Timothy B. Lueder; Richard A. Palmer; David M. Talcott; Lou Cheng Zhang; Mamoru Tachikawa; Toshio Saruyama; Kazutoshi Okabe
Original assignee: Dow Corning SA; Dow Corning Toray Silicone Co Ltd; Dow Corning Asia Ltd; Dow Corning Corp
Current assignee: Dow Silicones Belgium SPRL; DuPont Toray Specialty Materials KK; Dow Silicones Corp
Priority date: 2000-01-06
Filing date: 2001-01-04
Publication date: 2001-07-12
Anticipated expiration: 2002-07-06
Also published as: AU2473401A; WO2001049774A3

Abstract

A composition formed by mixing (i) a compound having the general formula M [OR] 4, where M represents a metal having a valence of 4 selected from Group IVB of the Periodic Table, and each R is the same or different and is selected from methyl and tertiary aliphatic hydrocarbon groups; with (ii) a compound of the general formula (1) wherein R1 is selected from a methylene group or a substituted methylene radical having 1 to 6 carbon atoms, A represents -(CX¿2?)nC(R?2)¿3, wherein n is from 0 to 5, each X is the same or different and is selected from a halogen radical and hydrogen, each R2 is the same or different and is selected from the group of a halogen radical and an alkyl radical having 1 to 8 carbon atoms; and B has the general formula OR3, where R3 is selected from methyl, ethyl, t-butyl, and amyl groups is disclosed as a catalyst for a moisture curable composition. The resulting moisture curable composition has improved green strengh and working time.

Description

ORGNNOSILOXANE COMPOSITIONS

This invention relates to moisture curable organosiloxane compositions that are curable to elastomeric solids and the use of such compositions as sealing materials. In particular, this invention relates to moisture curable organosiloxane compositions with both improved green strength and working time.

Organosiloxane compositions that cure to elastomeric solids are well known. Typically such compositions are obtained by mixing a polydiorganosiloxane having reactive terminal groups, with a silane cross-linking agent that is reactive with the terminal groups of the polydiorganosiloxane. Such silane groups include, for example an acetoxysilane, an oximosilane, an aminosilane or an alkoxysilane. These materials are frequently curable upon exposure to atmospheric moisture at room temperature.

One important application of the above-described curable compositions is their use as sealants. In use as sealants, it is important that the composition is capable of curing in comparatively thick layers to provide an elastomeric body having a thickness greater than about 2 mm. It is frequently desirable that the composition cures quickly enough to provide a sound seal within several hours but not so quickly that the surface cannot be tooled to desired configuration shortly after application, or so quickly that the surface cannot adhere to a second substrate shortly after application to a first substrate.

Among specifically desirable attributes for such compositions are fast surface cure rate, and good elasticity of the skin formed. In addition, there is a need to have a sealant that quickly develops green strength to allow the handling of assembled components during and soon after they are manufactured. Many of the sealant compositions used in the past, such as those silicone sealant compositions that evolve acetic acid, ketoximes, or amines may cause corrosion of sensitive metals or plastics. These sealant compositions also typically have some unpleasant associated odor during the curing process, especially when used in confined areas where assemblies are manufactured, such as in buildings used to manufacture such assemblies, for example insulated glass window units. The use of compositions catalyzed by titanium compounds and liberating alcohols upon cure are considered to be less corrosive and emit a more pleasant odor during the curing process than those which give off acetic acid, ketoximes, or amines. The need is therefore to have sealant compositions which will quickly develop green strength for the purpose of maintaining the manufactured assembly's integrity, are less corrosive, and are compatible with more of the materials used in the construction of assemblies. For the purposes of this invention, a composition which rapidly develops green strength is one which exhibits a fast-forming, high-modulus skin of sufficient strength such that elements of a construction can be formed and will maintain their desired configuration even if handled, packaged, and shipped after relatively short cure times, without showing permanent deformation. In order to achieve the desired speed of cure of alkoxysilane cured silicone compositions it has become a practice to employ certain organic titanium compounds as catalysts for the condensation reaction. Some of these titanium compounds are apt to react with methoxysilane to form a white precipitate, which restricts the ability of the composition to cure. The titanium compounds most generally preferred for this purpose are those derived from primary or secondary alcohols, for example, isopropyl alcohol and n-butyl alcohol. US 3334067 describes a method of preparing a one package room temperature silicone rubber by mixing an hydroxylated siloxane polymer with a silane cross-linking agent and a beta-dicarbonyl titanium compound such as bis (acetylacetonyl) diisopropyl titanate. EPO 164470 describes an organopolysiloxane fluid containing at least two alkoxysilyl organic radicals and titanium, zirconium, hafnium metal or vanadium oxide esters soluble in the liquid. EP0361803 describes a method for the in situ formation of titanium chelate catalysts in which a silicone sealant is prepared by adding an alkoxy or hydroxy endblocked polydiorganosiloxane, an alkoxy functional cross-linker and a titanate catalyst. In this case the titanate catalyst is a tetraorgano titanate such as tetra isopropoxy titanate and tetra butoxy titanate to which ethylacetoacetate is added. In EP0802222 there is described a method of improving the adhesion of a room temperature vulcanizable silicone composition comprising a polydiorganosiloxane having at least two alkoxy end groups, a cross linker filler and a titanium catalyst, for example tetraalkoxy titanates such as tetraethyl titanate and tetra isopropoxy titanate or chelated titanium compounds such as bis-acetylacetonyldiisopropoxy titanate, with 0.25 to 3 moles of monoketoester per mole of each titanium atom in the catalyst.

EP0747443, teaches the use of a catalyst according to the general formula M[OR]_x [OR'] _y where M represents a metal having a valence of 4 selected from Group IVB of the Periodic Table, x has a value from 0 to 0.60, y has a value from 3.40 to 4.0 and (x + y) = 4, and R' represents a monovalent, tertiary or branched secondary aliphatic hydrocarbon group and R represents a monovalent linear aliphatic hydrocarbon group having 1 to 6 carbon atoms. The catalyst was preferably tetra (tertiary butoxy) titanate or tetra (tertiary amyloxy) titanate. Such a catalyst used for the preparation of silicone sealants was said to cause fast curing compositions, without the need for a chelating agent.

SUMMARY OF THE INVENTION This invention is a metal chelate catalyst for use in room temperature curing compositions. The room temperature curing compositions of this invention are improved over earlier compositions in having both a longer working time and good green strength.

DESCRIPTION OF THE INVENTION The present invention provides in a first aspect a mixture and/or reaction product of a compound (i) has the general formula M [OR] ₄ , where M represents a metal having a valence of 4 selected from Group IVB of the Periodic Table, and each R is the same or different and is selected from methyl and tertiary aliphatic hydrocarbon groups; a compound ii) of the general formula

O O

II II

wherein R¹ is selected from a methylene group or a substituted methylene radical having 1 to 6 carbon atoms, A represents -(CX₂)_nC(R²)₃ wherein n is from 0 to 5, each X is the same or different and is selected from a halogen radical and hydrogen, each R² is the same or different and is selected from the group of a halogen radical and an alkyl radical having 1 to 8 carbon atoms; and B has the general formula OR³, where R³ is selected from methyl, ethyl, t-butyl, and amyl groups.

Regarding compound (i), the Group IVB elements of the Periodic Table in accordance with this invention are Titanium (Ti), Zirconium (Zr) and Hafnium (Hf). While M may represent any of the Group IVB elements, the preferred elements are titanium and zirconium. Each R group may be any primary alkyl group having 1 to 6 carbon atoms for example, methyl, ethyl, butyl, propyl, pentyl or hexyl groups, a secondary alkyl such as isopropyl, or tertiary alkyl groups such as C(CH₃)₃ or C(C H₅)(CH₃) . Preferably at least a majority and most preferably at least 75% of R groups are tertiary alkyl groups. One example of an appropriate compound (i) is sold as Tyzor 9000 by Dupont and has the formula Ti [isopropoxy] ^tertiary butoxy]_bZ where a is about 10% and b is about 90% and the total number of [isopropoxy] + [tertiary butoxy] groups per Ti atom is 4. The preparation of compounds of the above type is discussed in EP0747443. Regarding compound (ii), R¹ is most preferably a methylene group. In the group A, preferably at least one and more preferably each X is hydrogen. It is preferred that at least one and most preferably each R² group in A is hydrogen. In a most preferred formulation n is zero.

B has a general formula OR where R is selected from methyl, ethyl, t-butyl, and amyl groups.

Examples of compound (ii) include methylacetoacetate (MAA), ethylacetoacetate (EAA), tertiary butylacetoacetate (TBAN),

Methyl pivaloylacetate (MPN), otherwise known as Pentanoic acid, 4,4-dimethyl-3- oxo-, methyl ester, which has the following formula:

O O t-Bu C CH₂ C OMe

otherwise known as Butanoic acid, and Ethyl 4,4,4-trifluoroacetoacetate (TFN), 4,4,4- trifluoro-3-oxo-, ethyl ester, which has the following formula: - O O

F, ₃C - ^ CH- -₂ OEt

It is to be noted that the structures of compound (ii) are tautomeric by structure and therefore the above structures are general and are in real terms a combination of tautomers, and it is this tautomerism which enables compound (ii) to become a chelate of M when mixed/reacted with compound (i) wherein tautomers of compound (ii) may replace any OR. Hence, preferred reaction products in accordance with this first aspect of the invention may be depicted as follows:

However it is to be understood that in most instances there will be present a mixture and/or reaction product of all different forms of compounds (i) and (ii) including unreacted (i) and (ii) and the above from where p is 0 to p is 4. For M = titanium, p=2 or 3.

The molar ratio of compound (ii) to compound (i) is preferably no greater than 4 : 1, more preferably the ratio is between about 0.5 : 1 and 2 : 1, but most preferably it is in the region of 1 :1 to 2:1.

In a second aspect of the present invention there is provided a moisture curable composition capable of cure to an elastomeric body, the composition comprising a polymeric material having not less than two groups bonded to silicon which are hydroxyl or hydrolyzable groups, at least one alkoxysilane crosslinking agent and a catalyst in the form of the mixture and/or reaction product as described above made before adding. Also included within the scope of this invention are the cured elastomeric products of the said compositions and the use of such compositions for sealing joints, cavities and the like.

In a composition according to the second aspect of the invention the polymeric material is preferably a polydiorganosiloxane comprising on average at least 1.2 chain terminations per molecule selected from the group consisting of hydroxysilyl groups described by formula

-SiR4₂OH, (I) and alkoxysilyl groups described by formula -Z_y-SiR4_χ(OR5)3__χ, (II) where each R^ is independently selected from the group consisting of a hydrogen atom and monovalent hydrocarbon radicals comprising 1 to about 18 carbon atoms, each R^ is an independently selected alkyl radical comprising 1 to about 8 carbon atoms, each Z is independently selected from the group consisting of divalent hydrocarbon radicals comprising about 2 to 18 carbon atoms and a combination of divalent hydrocarbon radicals and siloxane segments described by formula R4 R4

-G-(SiO)_c-Si-G-

I I

R4 R4

where R4 is as defined above, each G is an independently selected divalent hydrocarbon radical comprising about 2 to 18 carbon atoms, and c is a whole number from 1 to about 6, x is 0 or 1, and y is 0 or 1.

More preferably, a second polydiorganosiloxane is used, comprising on average at least 1.2 chain terminations per molecule selected from the group consisting of hydroxysilyl groups, alkoxysilyl groups, and multi-alkoxysilyl groups described by the formula - Z_b-R6(Z-SiR4_n(OR5)₃„_n)_a (in) provided that at least one chain termination per molecule comprises a multi-alkoxysilyl group, where R , Rβ, and Z are as defined above, R° is independently selected from the group consisting of a silicon atom and a siloxane radical comprising at least two silicon atoms and each Z is bonded to a silicon atom of R^ with the remaining valences of the silicon atoms of R being bonded to a hydrogen atom, a monovalent hydrocarbon radical comprising 1 to about 18 carbon atoms, or forming siloxane bonds, n is 0, 1, or 2, a is at least 2, and b is 0 or

1, provided, when b is 0, R4 is bonded to the polydiorganosiloxane through a siloxane bond.

In a composition according to the invention each alkoxysilane crosslinking agent is of the general formula R⁴ ₄.qSi(OR⁵)_q wherein R⁴ and R⁵ are as described above, and q has a value of 2, 3 or 4. Preferred silanes are those wherein R⁴ represents methyl, ethyl or vinyl or isobutyl, R⁵ represents methyl or ethyl and q is 3. Examples of operative silanes are methyltri(methoxy)silane (MTM), vinyltrimethoxysilane, methyltriethoxysilane, and vinyltriethoxysilane, isobutyltrimethoxysilane (iBTM). A sufficient amount of this silane is employed to ensure adequate stability of the composition during storage and adequate crosslinking of the composition when exposed to atmospheric moisture.

Preferably the composition comprises 100 parts by weight of polymeric material, 0.1 to 20 parts by weight alkoxysilane crosslinking agent, and 0.1 to 10 parts by weight of catalyst in the form of the mixture and/or reaction product defined above. Compositions of this invention may contain as optional constituents other ingredients that are conventional to the formulation of silicone rubber sealants and the like. For example, the compositions will normally contain one or more finely divided, reinforcing or extending fillers such as high surface area fumed and precipitated silicas, crushed quartz, diatomaceous earths, calcium carbonate, barium sulfate, iron oxide, titanium dioxide and carbon black. The proportion of such fillers employed will depend on the properties desired in the elastomer- forming composition and the cured elastomer. Usually the filler content of the composition will reside within the range from about 5 to about 150 parts by weight per 100 parts by weight of the polymeric material. Other ingredients that may be included in the compositions are pigments, plasticizers, agents (usually organosilicon compounds) for treating fillers, rheological additives for improving tool -ability of the composition, such as silicone glycols, and adhesion improving substances, for example, γ-ammopropyltriethoxysilane, alone or in combination with γ- glycidoxypropyltrimethoxysilane. One conventional ingredient which can be employed as a plasticizer and to reduce the modulus of the cured elastomer is a polydimethylsiloxane having terminal triorganosiloxy groups wherein the organic substituents are e.g. methyl, vinyl or phenyl or combinations thereof. Such polydimethylsiloxanes normally have a viscosity of from about 100 to about 100,000 mPa- s at 25°C and can be employed in amounts up to about 80 parts per 100 parts by weight of the polymeric material. Alternative plasticizers may include organic plasticizers, which will be well known to the person skilled in the art such as di-octyl phthalate and petroleum distillates.

A further additive which may be introduced into the system, preferably at the same time as compound (ii) is (tertiary amyl) alcohol, this is particularly the case where compound (i) comprises isopropoxy groups as the isopropoxy groups are readily labile on hydrolysis and alcoholysis and are rapidly exchanged with the (tertiary amyloxy) groups. Hence, for example when compound (i) is 2 % by weight of Ti [isopropoxy]_az[tertiary butoxy]_bZ where a is about 10% and b is about 90% and the total number of [isopropoxy] + [tertiary butoxy] groups per Ti atom is 4 (Tyzor 9000), compound (ii) might be 0.1 to 1.0 % by weight of ethyl 4,4,4-trifluoracetoacetate TFA which could be added simultaneously with 0.2 to 0.5 % by weight of (tertiary amyl) alcohol.

In a third aspect of the invention there is provided a method of forming a composition in accordance with the second aspect of the invention, comprising mixing at least one of the polymeric material or a crosslinking agent together with compound (i) and compound (ii) such that the mixture and/or reaction product of compound (i) and compound (ii), as defined in the first aspect of this invention, is formed in situ.

In accordance with the third aspect of the invention the compositions may be prepared by mixing the ingredients in any order and employing any suitable mixing equipment. It is generally preferred to form the mixture and/or reaction product of compound (i) and compound (ii) after mixing together the polymeric material and the crosslinking agent silane. However, it is also possible to add compound (i) to the polymeric material and the crosslinking agent silane, mixing the three together and then finally adding compound (ii) thereafter for an alternative in-situ preparation of the mixture and/or reaction product of compound (i) and compound (ii). In a further alternative compound (i) and Compound(ii) may be added into a premix with one or more cross-linkers prior to addition of the pre-mix into the polymeric material. Any optional additional ingredients other than the filler may be incorporated at any stage of the mixing operation but are preferably added after the catalyst is formed. Compound (i) and the crosslinking agent silane are preferably added to the polymer prior to the introduction of any filler; but compound (ii) may be added either before, simultaneously, or after the introduction of the filler.

After mixing, the compositions may be stored under substantially anhydrous conditions, for example in sealed containers, until required for use. ι

Compositions according to the invention may be formulated as single part formulations which are stable in storage but cure on exposure to atmospheric moisture and may be employed in a variety of applications, for example as coating, caulking and encapsulating materials. They are, however, particularly suitable for sealing joints, cavities and other spaces in articles and structures which are subject to relative movement. They are thus particularly suitable as glazing sealants, sealants for insulating glass units used in windows, and for sealing building structures. They have desired cure properties to provide cured seals of modulus sufficiently low for most industry standards and elongation to break that is sufficiently high for most industry standards.

In order that the invention may become clearer there now follows a description of example sealant compositions selected for description to illustrate the invention by way of example. In the description all compositions are expressed by weight % and all viscosities are at 25°C unless otherwise indicated. EXAMPLES INGREDIENTS USED IN EXAMPLES

Acac = Acetyl acetone Adhesion Promoter = 2:1 molar mixture of Glycidoxypropyltrimethoxysilane and Aminopropyltrimethoxysilane.

EAA = Ethyl acetoacetate Filler 1 = Untreated fumed silica with 160 +/- 15 m²/g surface area (LM-150D, Cabot Corporation, Tuscola, IL).

Filler 2 = Ground calcium carbonate treated with stearate with a particle size = 3 microns (CS-11, Cabot Corporation, Tuscola, IL).

MAA = Methyl acetoacetate

MPA = Methyl pivaloylacetate MTM = Methyltrimethoxysilane

Plasticizer = Trimethyl end-blocked polydimethylsiloxane fluid having a viscosity of 0.1 Pa^' s

Polymer = Trimethoxysilethylene end-blocked polydimethylsiloxane having a viscosity of 65 Pa s

Rheology Additive = Dimethyl, Methyl(polypropylene oxide) Siloxane having a viscosity of 0.2 Pa^' s TBAA = t-Butyl acetoacetate

TiPT = Tetra-isopropyl Titanate

Titanate Chelate 1 = Di(ethylacetoacetate)-diisopropoxy titanate, purchased from E. I. du Pont de Nemours and Company, Wilmington DE

Titanate Chelate 2 = Di(methylacetoacetate)-ditertbutoxy titanate

Titanate Chelate 3 = Di(methylacetoacetate)-dimethoxy titanate

TtBT = Tetra-t-Butyl Titanate

TEST METHODS

Skin-Over-Time (SOT) represents the amount of time for the surface of a curable composition to form a skin of cured material. The SOT affects how long end-user can take to join a first member having an applied bead of the composition to a second member without having the formation of a layer of cured material or "skin" on the surface interfere with adhesion to the second member. Skin-Over-Time was measured by spreading the material to form a layer 0.32+/-0.08 cm thick on a clean smooth non-porous surface. The sample was exposed to a relative humidity (RH) of 50% at 22°C and at one-minute intervals the surface was lightly touched with a fingertip and the finger slowly drawn away. This was repeated every minute until the sample did not adhere to the fingertip. The time in minutes elapsed from spreading the material until the surface did not adhere to the fingertip was recorded as Skin-Over-Time.

Green strength is a measure of the sealant' s strength during cure, but prior to complete cure. Persons skilled in the art commonly use instruments generically known as dynamical mechanical rheometers for characterizing the strength of a curing composition. In the following examples, green strength was quantified by measuring the sheer strength after 60 minutes of sealant cure using a dynamic mechanical rheometer equipped with a parallel plate sample holder (Rheometrics®, model RDS 7700). The measurements were made in accordance with ASTM D2084-95 with the main difference being that the sample holding fixture had smooth plates instead of the ribbed plates used in the ASTM method. Sealant compositions were pressed between two 25 mm parallel plates such that a gap of 1 mm was obtained between the plates. After 60 minutes of curing at 50% RH at 22°C, the shear strength of the sealant was measured by means of a torque transducer at 5% strain and oscillating frequency of 1 rad/s and is reported as green strength in units of Pascals, Pa. Samples were analyzed after initial room temperature aging and after aging for 1 week at 50°C in a cartridge.

Preparation of Titanate Chelate 2 The Titanate Chelate 2 was prepared by the reaction of tetra-tertiary-butoxy titanate (TtBT) and methyl acetoacetate (MAA). To a three-neck flask fitted with an addition funnel, a thermometer, and a condenser with a vacuum line, 110 grams (0.32 mol) of TtBT was added. The MAA (75 g, 0.65 mol) was added dropwise by use of the addition funnel while stirring with a magnetic stir bar. The addition took 0.5 hours and the temperature rose from 23 C to 56 C. After completion of the addition, the solution was stripped under reduced pressure. The volatiles were collected in a cold trap and analyzed to yield 46.1 grams (0.62 mol) of t- butanol. Preparation of Titanate Chelate 3

The Titanate Chelate 3 was prepared by the reaction of tetra-isopropoxy titanate (TiPT) and methyl acetoacetate followed by alcohol exchange, using methanol. To a three-neck flask fitted with an addition funnel, a thermometer, and a distillation condenser fitted with a receiving flask and a vacuum line, 223 grams (0.79 mol) of TiPT was added. The MAN (184 g, 1.59 mol) was added dropwise by use of the addition funnel while stirring using a magnetic stir bar. The addition took approximately 45 minutes and the temperature rose from 23 C to 50 C. After completion of the addition, the solution was stripped under reduced pressure. The volatiles were collected in a cold trap and analyzed to yield 71.9 grams (1.20 mol) of isopropanol. In three separate additions, 213 grams (6.66 mol) of methanol was then added dropwise by use of the addition funnel while simultaneously removing any alcohols generated by distillation (Vapor Temp = 40C, Pressure = 125 mm Hg). The volatiles from the distillation were collected in the receiver flask and analyzed to yield 92.1 grams (1.5 mol) isopropanol and 165 grams (5.2 mol) of the excess methanol.

Example 1

Sealant compositions were prepared by mixing 42.0-42.5% of Polymer, 10% of Plasticizer, 0.5% of MTM, 2.0-2.5% of titanate catalyst, 3.5% of Filler 1, 40% of Filler 2, 0.5% of a rheology additive, and 1.0% of Adhesion Promoter, at room temperature in absence of moisture. The titanate catalysts were kept at equal molar levels and the Polymer was adjusted to compensate for any differences in level. Unless otherwise noted, 100 g quantities were prepared for all batches.

Samples were prepared using a small batch mixer (Whip Mix® Corporation) equipped with a vacuum line. In the first step, Polymer, Plasticizer, MTM, Adhesion Promoter, Rheology Additive, and titanate were mixed for 1 minute. The fillers were subsequently added and the resulting mixture was dispersed for about 1 -minute followed by a scrape-down and another 1 -minute mix. The final composition was de-aired at 50 mm Hg vacuum at the end of the compounding process for one minute prior to packaging the sealant into Semco® polyethylene cartridges. After packaging, the cartridges were then centrifuged to remove any entrapped air from the packaging. The composition was then matured for overnight in its packaging under ambient conditions of the lab before evaluating the curing properties known as Skin-Over-Time (SOT) and Green Strength. Table 1 presents the formulations and test results.

Example 2

To demonstrate the utility of various chelating agents in a fast-cure formulation, a series of experimental runs were completed. Table 2 shows the results from a series of experimental runs that compare the use of various tetra-functional titanates and chelating agents in a fast-cure sealant formulation. The compositions consisted of 41.1-42.2% by weight Polymer, 10% of Plasticizer, 0.5% MTM, 3.5% of Filler 1, 40% of Filler 2, 0.5% of Rheology Additive, 1.0% of Adhesion Promoter, titanate of the formula Ti [OR] ₄ at the level shown in Table 2, and a chelating agent at the molar ratios shown in Table 2. The titanate catalysts were kept at equal molar levels and the Polymer was adjusted to compensate for any differences in level. The sealants were prepared, packaged, and tested as described in Example 1.

Table 2: The effect of various chelated titanate com ounds on a fast-cure sealant formulation.

Claims

We claim:

1. A composition formed by mixing:

(i) a compound having the general formula M [OR] , where M represents a metal having a valence of 4 selected from Group IVB of the Periodic Table, and each R is the same or different and is selected from methyl and tertiary aliphatic hydrocarbon groups; (ii) a compound of the general formula

O O

II II

2. The composition of claim 1 where M is selected from titanium and zirconium.

3. The composition of claim 1 where 75 molar percent of R groups are tertiary alkyl groups.

4. The composition of claim 3 where the tertiary alkyl groups are selected from

C(CH₃)₃ and C(C₂H₅)(CH₃)₂ .

5. The composition of claim 1 where R¹ is a methylene group.

6. The composition of claim 1 where R² is hydrogen.

7. The composition of claim 1 where n is zero.

8. The composition of claim 1 where (ii) is selected from methylacetoacetate, ethylacetoacetate, tertiary butylacetoacetate, methyl pivaloylacetate , and Ethyl 4,4,4-trifluoroacetoacetate.

9. The composition of claim 1 where the molar ratio of (ii) to (i) is no greater than 4:1.

10. The composition of claim 1 where the molar ratio of (ii) to (i) is between 0.5:1 and 2:1.

11. The composition of claim 1 where (i) is selected from (t-BuO)₄Zr and (t-BuO) Ti.

12. A moisture curable composition formed by mixing ingredients comprising: a. a polymeric material having not less than two groups bonded to silicon which are hydroxyl or hydrolyzable groups, b. at least one alkoxysilane crosslinking agent, c. the composition of claim 1.

13. The composition of claim 12 where the polymeric material a polydiorganosiloxane comprising on average at least 1.2 chain terminations per molecule selected from the group consisting of hydroxysilyl groups described by formula -SiR⁴ ₂OH, (I) and alkoxysilyl groups described by formula

-Z_y-SiR4_χ(OR5)3__χ, (II) where each R4 is independently selected from the group consisting of a hydrogen atom and monovalent hydrocarbon radicals comprising 1 to about 18 carbon atoms, each R^ is an independently selected alkyl radical comprising 1 to about 8 carbon atoms, each Z is independently selected from the group consisting of divalent hydrocarbon radicals comprising about 2 to 18 carbon atoms and a combination of divalent hydrocarbon radicals and siloxane segments described by formula

R4 R I I

-G-(SiO)_c-Si-G-

I I

R4 R4

14. The composition of claim 13, wherein the polymeric material further comprises a second polydiorganosiloxane is used, comprising on average at least 1.2 chain terminations per molecule selected from the group consisting of hydroxysilyl groups, alkoxysilyl groups, and multi-alkoxysilyl groups described by the formula - Z_b-R6(Z-SiR4_n(OR5)₃__n)_a (III) provided that at least one chain termination per molecule comprises a multi- alkoxysilyl group, where R4, R$, and Z are as defined above, R^ is independently selected from the group consisting of a silicon atom and a siloxane radical comprising at least two silicon atoms and each Z is bonded to a silicon atom of R° with the remaining valences of the silicon atoms of R" being bonded to a hydrogen atom, a monovalent hydrocarbon radical comprising 1 to about 18 carbon atoms, or forming siloxane bonds, n is 0, 1, or 2, a is at least 2, and b is 0 or 1, provided, when b is 0, R4 is bonded to the polydiorganosiloxane through a siloxane bond.

15. The composition of claim 12, wherein the alkoxy silane has the general formula R⁴ ._qSi(OR⁵)q wherein R⁴ and R⁵ are as described above, and q has a value of 2, 3 or

4. Preferred silanes are those wherein R⁴ represents methyl, ethyl or vinyl or isobutyl, R⁵ represents methyl or ethyl and q is 3.

16. The composition of claim 12 where the proportions of components a, b, and c are 100 parts by weight component a, 0.1 to 20 parts by weight component b, and 0.1 to 10 parts by weight component c.

17. A method of making a moisture curable composition comprising first mixing together a polymeric material having not less than two groups bonded to silicon which are hydroxyl or hydrolyzable groups, and at least one alkoxysilane crosslinking agent, then adding

(i) a compound having the general formula M [OR] ₄ , where M represents a metal having a valence of 4 selected from Group

IVB of the Periodic Table, and each R is the same or different and is selected from methyl and tertiary aliphatic hydrocarbon groups;

(ii) a compound of the general formula o o

II II

A- C - Ri - C - B wherein R¹ is selected from a methylene group or a substituted methylene radical having 1 to 6 carbon atoms, A represents -(CX₂)_πC(R²)₃ wherein n is from 0 to 5, each X is the same or different and is selected from a halogen radical and hydrogen, each R² is the same or different and is selected from the group of a halogen radical and an alkyl radical having 1 to 8 carbon atoms; and

B has the general formula OR³, where R is selected from methyl, ethyl, t-butyl, and amyl groups.