MOISTURE CURING ELASTOMER COMPOSITIONS

[0001] This invention relates to moisture curing elastomers curing by the reaction of hydrolysable silyl groups. Such elastomers are widely used as one part or two part moisture curing sealants and adhesives. The elastomers are based on an organic or siloxane polymer carrying hydrolysable groups curable through silyl chemistry. By an organic polymer we mean a material based on carbon chemistry that is a polymer in which at least half the atoms in the polymer backbone are carbon atoms.

[0002] Moisture curing sealants based on polyorganosiloxanes curing by the reaction of hydrolysable silyl groups are described for example in US-A-4797446, US-A-4898910, EP- A-1254192, EP-A-368500, EP-A-438222, WO-A-2001/49774 and GB-A-1284203. Organic polymers curing by the reaction of hydrolysable silyl groups and useful as adhesives or sealants are described for example in EP-A-1746133, EP-A-1746134 and EP-A-1746135.

[0003] Historically the most favoured condensation catalysts for the cure of such formulations are organotin compounds with an oxidation state of (IV). Whilst these are very efficient catalysts they have been identified as potentially problematic from a safety and/or environmental perspective.

[0004] Several moisture curing elastomers which cure using with aid of titanate catalysts these include GB1306935, JP2001-261915, JP2000-344982, US2005/0165169, US64691 15 and US4578417, however none of these identify or even remotely suggest that there is a significant enhancement in elastic recovery caused specifically by the use of the catalyst.

[0005] The inventors have found that a number of potential replacements for tin (IV) catalysts for use in curing the above provide the resulting cured sealants with a significant physical improvement over sealants cured using tin (IV) based catalysts, namely a significantly improved elastic recovery.

[0006] According to the invention a titanate, zirconate, tin (II) and/or aluminate catalyst is used to improve the elastic recovery of a moisture curing elastomer curing by the reaction of hydrolysable silyl groups.

[0007] In an alternative embodiment there is provided a method comprising: i) adding a catalyst selected from a titanate catalyst, a zirconate catalyst, a tin (II) catalyst or an aluminate catalyst to a moisture curable elastomer composition, and ii) curing the product of step i) by the reaction of hydrolysable silyl groups with the catalyst to form a moisture cured elastomer; thereby improving the elastic recovery of the moisture cured elastomer.

[0008] There is also provided a method for improving the elastic recovery of a moisture cured elastomer curing by the reaction of hydrolysable silyl groups characterized by use of a titanate, zirconate, and/or tin (II) catalyst.

[0009] Elastic recovery is a measure of the recovery of a sample after being stressed or stretched over a predetermined distance for a predetermined period of time and identifying the level of recovery (usually measured as a percentage) of the original shape after a set period of time (typically 1 hour or 24 hours). We have found that the use of titanate zirconate or tin (II) catalysts improves the elastic recovery and movement capability of the moisture curing elastomer compared to the organotin (IV) catalysts generally used for catalyzing the moisture cure of organic polymers curing by the reaction of hydrolysable silyl groups. The titanate, zirconate and/or tin (II) catalyst is shown to improve the elastic recovery of a cured product obtainable from a moisture cured (or curable) elastomeric composition, by the reaction of hydrolysable silyl groups with the catalyst. The improvement of the elastic recovery enhances the movement capability of the cured elastomer, enabling the use of these elastomers as an improved weather sealant.

[0010] The inventors believe that the improvement is caused because in principle of the catalysts described above as well as others such as aluminates), the tin (IV) catalysts are the only catalysts that remain chemically unchanged subsequent to curing the sealant composition. Hence, the tin (IV) catalysts remain active as a hydrolysis and condensation catalyst within the cured sealant. When subsequently stretched therefore, sealants containing tin (IV) catalysts cause Si-O-Si bond cleavage and recombination, leading to poor elastic recovery of the system. However, the catalysts used in accordance with the present invention, i.e. titanates, zirconates, and tin (II) catalysts (as well as others such as aluminates) gradually decrease in activity with time and exposure to water and oxygen and as such cannot activate the cleavage of Si-O-Si bonds.

[0011] Examples of suitable titanium compound catalysts include titanium alkoxides, otherwise known as titanate esters. Zirconium alkoxides (zirconate esters) can alternatively be used. Titanate and/or zirconate based catalysts may comprise a compound according to the general formula Ti[OR ⁵ J ₄ and Zr[OR ⁵ J ₄ respectively where each R ⁵ may be the same or different and represents a monovalent, primary, secondary or tertiary aliphatic hydrocarbon group which may be linear or branched containing from 1 to 10 carbon atoms. Optionally the titanate may contain partially unsaturated groups. However, preferred examples of R ⁵ in titanates include but are not restricted to methyl, ethyl, propyl, isopropyl, butyl, tertiary butyl, 2-ethylhexyl and a branched secondary alkyl group such as 2,4-dimethyl-3-pentyl.

Preferably, when each R ⁵ is the same, R ⁵ is an isopropyl, branched secondary alkyl group or a tertiary alkyl group, in particular, tertiary butyl.

[0012] Alternatively, the titanate may be chelated. The chelation may be with any suitable chelating agent such as an alkyl acetylacetonate such as methyl or ethylacetylacetonate. Any suitable chelated titanates or zirconates may be utilised. Preferably the chelate group used is a monoketoester such as acetylacetonate and alkylacetoacetonate giving chelated titanates such as, for example diisopropyl bis(acetylacetonyl)titanate, diisopropoxy bis(ethylacetoacetate)titanate, diisobutoxy bis(ethylacetoacetate)titanate and the like, or the catalyst can be octylene glycol titanate. Examples of suitable catalysts are additionally described in EP1254192 and WO2001/49774 which are incorporated herein by reference.

[0013] Preferred zirconate catalysts include tetra-n-propyl zirconate, tetra-n-butyl zirconate and zirconium diethylcitrate.

[0014] Suitable tin (II) catalysts for the invention include for example bis ( 2- ethylhexanoate) tin; bis (neodecanoate) tin; stannous acetate, tin oxalate, tin (II) 2,4- pentanedionate, tin oleate, tin naphthate, tin butyrate, tin acetate, tin benzoate, tin sebacate, and tin succinate.

[0015] The amount of the titanate or zirconate and/or tin (II) catalyst can for example be 0.01-5% based on the weight of the moisture curing polymer plus crosslinking agent.

[0016] The moisture curing elastomer curing by the reaction of hydrolysable silyl groups can be based on an organic polymer or on a siloxane polymer. The organic or siloxane

polymer preferably carries hydrolysable silyl groups. As an alternative the polymer can carry groups such as hydroxyl groups which react with hydrolysed silyl groups, in which case it is used with a crosslinker containing hydrolysable silyl groups. We have found particularly improved elastic recovery for moisture curing elastomers based on organic polymers containing hydrolysable silyl groups.

[0017] The hydrolysable silyl groups preferably contain alkoxy groups bonded to silicon, although alternative hydrolysable groups such as acetoxy can be used. The hydrolysable silyl groups can for example be dialkoxyalkylsilyl groups, dialkoxyalkenylsilyl groups or trialkoxysilyl groups. Dialkoxyalkylsilyl or dialkoxyalkenylsilyl groups of the formula -SiR'(OR) ₂ , in which R represents an alkyl group having 1 to 4 carbon atoms, most preferably methyl or ethyl, and R' represents an alkyl or alkenyl group having 1 to 6 carbon atoms, are particularly preferred. Examples of such dialkoxyalkylsilyl groups are dimethoxymethylsilyl, diethoxymethylsilyl and diethoxymethylsilyl groups. Examples of dialkoxyalkenylsilyl groups are dimethoxyvinylsilyl and diethoxyvinylsilyl. Examples of trialkoxysilyl groups are trimethoxysilyl and triethoxysilyl.

[0018] One preferred type of organic polymer having hydrolysable silyl groups is an acrylate polymer. The acrylate polymer is an addition polymer of acrylate and/or methacrylate ester monomers, which comprise at least 50% by weight of the monomer units in the acrylate polymer. Examples of acrylate ester monomers are n-butyl, isobutyl, n- propyl, ethyl, methyl, n-hexyl, n-octyl and 2-ethylhexyl acrylates. Examples of methacrylate ester monomers are n-butyl, isobutyl, methyl, n-hexyl, n-octyl, 2-ethylhexyl and lauryl methacrylates. The acrylate polymer preferably has a glass transition temperature Tg below ambient temperature; acrylate polymers are generally preferred over methacrylates since they form lower Tg polymers. Polybutyl acrylate is particularly preferred. The acrylate polymer can contain lesser amounts of other monomers such as styrene, acrylonitrile or acrylamide. The acrylate(s) can be polymerized by various methods such as conventional radical polymerization, or living radical polymerization such as atom transfer radical polymerization, reversible addition-fragmentation chain transfer polymerization, or anionic polymerization including living anionic polymerisation.

[0019] The organic polymer is preferably a telechelic polymer having terminal curable silyl groups, for example polybutyl acrylate having terminal curable silyl groups. The curable silyl groups can for example be derived from a silyl-substituted alkyl acrylate or methacrylate

monomer. Hydrolysable silyl groups such as dialkoxyalkylsilyl or trialkoxysilyl groups can for example be derived from a dialkoxyalkylsilylpropyl methacrylate or trialkoxysilylpropyl methacrylate. When the acrylate polymer has been prepared by a polymerisation process which forms reactive terminal groups, such as atom transfer radical polymerization, chain transfer polymerization, or living anionic polymerisation, it can readily be reacted with the silyl-substituted alkyl acrylate or methacrylate monomer.

[0020] The organic polymer can alternatively contain grafted, pendant or copolymerised curable silyl groups. For example a silyl-substituted alkyl acrylate or methacrylate monomer can be copolymerised with other acrylate monomers such as butyl acrylate, or an acrylate polymer containing pendant reactive groups can be reacted with a silyl compound having co- reactive groups.

[0021] Other suitable types of organic polymer having hydrolysable silyl groups include silyl modified polyisobutylene, silyl modified polyurethanes and silyl modified polyethers, which are all available commercially in the form of telechelic polymers. Silyl modified polyisobutylene can for example contain curable silyl groups derived from a silyl-substituted alkyl acrylate or methacrylate monomer such as a dialkoxyalkylsilylpropyl methacrylate or trialkoxysilylpropyl methacrylate, which can be reacted with a polyisobutylene prepared by living anionic polymerisation, atom transfer radical polymerization or chain transfer polymerization. Silyl modified polyurethanes or polyethers can for example be prepared by the reaction of polyurethanes or polyethers having terminal ethylenically unsaturated groups with a silyl monomer containing hydrolysable groups and a Si-H group, for example a dialkoxyalkylsilicon hydride or trialkoxysilicon hydride. The polyurethane or polyether having terminal ethylenically unsaturated groups can be prepared by reacting a hydroxyl-terminated polyurethane or polyether with an ethylenically unsaturated compound containing a group reactive with hydroxyl, for example an epoxide group.

[0022] The polymer having hydrolysable silyl groups can alternatively be a polyorganosiloxane, preferably a polydiorganosiloxane. The polydiorganosiloxane can for example have terminal groups of the formula -SiR'(OR) ₂ or -Si(OR) ₃ where R and R' are defined as above, and can be prepared by reacting a hydroxyl-terminated polydiorganosiloxane with an alkoxysilane of the formula R ¹ Si(OR) ₃ and/or a tetraalkoxysilane of the formula Si(OR) ₄ , in the presence of a catalyst for the condensation of silanol groups with Si-alkoxy groups. The hydroxyl-terminated polydiorganosiloxane from

which the polydiorganosiloxane of the invention is prepared can be a substantially linear polydiorganosiloxane, or can be a branched polydiorganosiloxane containing T units of the formula SiR"C>3/ ₂ , where R" represents an alkyl, substituted alkyl, alkenyl or aryl group, and/or Q units of the formula SiO _4/2 . The diorganosiloxane units are preferably dimethylsiloxane units. The diorganosiloxane units can additionally or alternatively to dimethylsiloxane units comprise methylphenylsiloxane units, alkylmethylsiloxane or dialkylsiloxane units in which the alkyl group has 2 or more, for example 2 to 12, carbon atoms, diphenylsiloxane units, methylvinylsiloxane units or methylaralkylsiloxane units.

[0023] A moisture curing elastomer based on a polymer having hydrolysable silyl groups such as dialkoxyalkylsilyl or trialkoxysilyl groups is generally self-curable, but it is often preferred that the elastomer composition contains a trialkoxysilane as crosslinker for the hydrolysable silyl groups. Examples of suitable trialkoxysilanes include methyltrimethoxysilane, ethyltrialkoxysilane, vinyltrimethoxysilane, methyltriethoxysilane and vinyltriethoxysilane. The trialkoxysilane crosslinker can for example be present at 0.1 to 15% by weight of the elastomer composition.

[0024] An alkyltrialkoxysilane in which the silicon bonded alkyl group is substituted by a polar functional group can act as both crosslinker for the elastomer composition and as an adhesion promoter. Examples of such silanes are aminosilanes such as 3- aminopropyltrimethoxysilane, 3-(2-aminoethylamino)propyltrimethoxysilane, 3- aminopropyltriethoxysilane or 3-aminobutyltrimethoxysilane, or mercaptosilanes such as 3- mercaptopropyltrimethoxysilane. Such an adhesion promoter can for example be present at 0.01 to 10% by weight of the elastomer composition.

[0025] The cross-linker may also comprise a disilaalkane of the formula:

R ¹ a R ⁴ _b

(R ² O) ₃ . _a Si-R ³ -Si(OR ⁵ ) ₃ . _b

where R ¹ and R ⁴ are monovalent hydrocarbons, R ² and R ⁵ are alkyl groups or alkoxylated alkyl groups, R ³ is a divalent hydrocarbon group and a and b are 0 or 1. Specific examples include 1 ,6-bis(trimethoxysilyl)hexane1 ,1-bis(trimethoxysilyl)ethane, 1 ,2- bis(trimethoxysilyl)ethane, 1 ,2-bis(trimethoxysilyl)propane, 1 ,1-

bis(methyldimethoxysilyl)ethane, 1 ,2-bis(triethoxysilyl)ethane,1-trimethoxysilyl-2- methyldimethoxysilylethane, 1 ,3-bis(trimethoxyethoxysilyl)propane, and 1- dimethylmethoxysilyl-2-phenyldiethoxysilylethane.

[0026] The polymer containing hydrolysable silyl groups can form up to 90% by weight of the elastomer composition but is preferably present at 10 to 60%. The elastomer composition can for example contain a plasticiser, a rheological agent to improve the flow properties of the sealant and/or one or more fillers. Examples of plasticizers include ester plasticizers such as phthalates, for example alkyl benzyl phthalates such as butyl benzyl phthalate or dialkyl phthalates such as dioctyl phthalate. The plasticizer can for example be present at 0 to 50% by weight of the elastomer composition, preferably 5 to 25%. Examples of rheological agents include those sold under the trade mark Polyvest ^® by Degussa GmbH. The rheological agent can for example be present at 0 to 5% by weight of the elastomer composition.

[0027] Examples of fillers include calcium carbonate, which can be precipitated calcium carbonate and/or ground calcium carbonate, rice hull ash, zeolites, or silica, including fumed silica, fused silica and/or precipitated silica. The filler can for example be present at 0 to 70% by weight of the elastomer composition, preferably 20 to 65%.

[0028] The moisture curing elastomers of the invention can be used as sealants. The improvement in elastic recovery enhances the movement capability of the sealant, resulting in improved durability on weathering and ageing. The moisture curing elastomers of the invention thus have particular advantage in sealants for demanding applications in which they are exposed to the weather and/or a particularly long life is required.

[0029] The invention is illustrated by the following Examples, in which percentages are by weight

EXAMPLE 1

[0030] Poly-n-butyl acrylate sealant containing 3-(methyldimethoxysilyl)propyl groups was mixed in a dental mixer with various amount of moisture curing catalysts.

[0031] Diisopropoxy bis(ethylacetoacetate)titanate catalyst (TDIDE) was mixed with methyltrimethoxysilane (MTMS) in weight ratio 4:1 to form a catalyst composition. The above sealant was mixed with various amounts of the catalyst composition for 2 x 30 seconds in a dental mixer and then poured into moulds to make 2 mm thick sheets. After 14 days of cure at 23 ⁰ C and 50% relative humidity (RH), dumbbells were cut in the sheets and subjected to 50% elongation for 24 hours. Dimensions were recorded after 24 hours of recovery. The elastic recovery results (%) are shown in Table 1.

[0032] In comparative experiments, the sealant composition was mixed with various amounts of each of the tin catalysts dibutyltin dilaurate (DBTDL), Bis((2- ethylhexyloxy)maleoloxy)dibutylstannane (BEMD) and di(n-butyl)tin bis-ketonate (DTBK) and cured under the conditions described above. The elastic recovery of the products of the comparative experiments is also shown in Table 1.

TABLE 1

[0033] As can be seen from Table 1 , the compositions cured with a titanate catalyst showed much superior elastic recovery compared to the compositions cured with a tin catalyst.

EXAMPLE 2

[0034] In this example a wider selection of both polymers in accordance with the present invention and catalysts were compared using the experimental protocol used in Example 1. All viscosity measurements were taken at room temperature (23 ⁰ C). The elastic recovery results (%) are shown in Tables 2a and 2b. It will be seen that there is a general pattern clearly showing that when comparing results using the same polymers it is found that the polymers cured using titanate or zirconate catalysts repeatedly, surprisingly gave the best elastic recovery results.

TABLE 2a

(209 Zr) is K-KAT XCA, a zirconium chelated catalyst from King Industries.

TABLE 2b

IBAY is diisobutoxy-bis(ethylacetoacetate) titanate