CA1071479A - Polyurethane-sealants and caulks - Google Patents

Abstract

ABSTRACT

This invention involves a new class of silane adhesion promoters for the bonding of polyurethane sealants to a variety of substrates. These silane adhesion promoters are bis-trialkoxysilylorgano polysulfide compounds.

Description

~ ~ 7 ~ ~ ~ 9 D-9827 Thls invention relates to an improvement in the bonding of polyurethane sealants and caulks ~o inorganic substrates such as glass, metals, metal oxides, mineral, and the like, in other words, the general class of inorganic oxide substrates. More particularly, this invention relates to the use of sllane adhesion prono~s con~
t~ining polysl~del;~ka~ ~her~m toe~Y~e ~e bo~ding of such polyurethanes to such substra~es.
There are a number of patents concerned with the bonding of polyurethanes to a variety of inorganic substrates using organo ~unctional silanes as adhesion promoters. For example, U~S. Patent 3,246,671, describes the use of certain amino silanes as adhesion promoting primers for clay pipe junctures to be sealed with urethane polymers. U.S. Patent 3,~53,136, u~ilized a polymeric methacrylate silane as an adhesion promoter for poly~
urethanes. U.S. Patent 3,779,79~, describes the use o a mercaptopropyltrimethoxysilane.
A rather good discussion on the use o silane adheslon promoters (i.e. silane coupling agents) can be ound in two articles by Swanson and Price, in i'Adhesive Age", on March 1972, at pages 26-32 and on June, 1973, at pages 23-28. A range of dif~erent silane adhesion promoters are characterized in the 1972 article and the ef~ectiveness of each is discussPd in the bonding of the urethane adhesives. In the conclusion of that arti~le,

2 -~07.1 479 Swanson and Price s~ated tha~ "prepolymer-silane reaction is necessary for high peel strengths after humidity aging." The 1972 article indicates that the prepolymer is a polyurethane polymer containing ~ree isocyanate or hydroxyl or amine gro~ps. The authors considered the aging stability when the silanes were mixed with the pre-polymer and from this concluded that certain silanes such as those containing "tertiary amine, epoxy and glycidoxy silanes---resulted in gelation of the isocyanate pre-polymer and thus were considered unusable for formulatingshelf-stable, two-component adhesive." The amine sllanes were found by the authors to "reach an unworkable vis-cosity at three weeks and gave indication of continuing reaction." Only the mercapto and chloroalkyl substituted silanes were consid~æ~ shel~stable as silane-prepolymer combinations."
One condition which Swanson and Pr~ce found critical is the reaction between polyurethane prepol~mers and the silane adhesion promoter. This suggests a signifi-cant restriction on the utilization o silane adhesionpromoters for polyurethane based sealants where the poly urethane is a ~ully polymerized typically ~hermoplas~ic material Such polyurethanes ha~e found signi~icant utility in the seal~nt and caulk mar~et. Having a silane adhesion promoter which is ef~ective with this klnd o~
polyuret~ane would be a most desirable and useful con-tribution to the ar~.

Silane adhesion promoters which ha~e been found ~o be consistently effective as primers for a variety of polymer polyurethanes based compositions are, for example, mercaptopropyltrimethoxysilane, beta-mercaptoethyltriethoxysilane, gamma-glycidoxypropyl-trlmethoxysilane, cohydrolyzates of gamma-aminopropyL-triethoxysllane and beta-(3, 4- epo~ycyclohexyl) ethyltrimethoxysilane, and cohydrolyzates of gamma-mercaptopropyltrimetho~ysilane and phenyl~riethoxysilarle.
There is herein described a process of bonding polyurethane sealants or caulks whlch optionally possess reactive isocyanate groups to a wide varlety of inorganic substrates with a select class of silane adhesion pro-moters More particularly, this invention in~olves providing on the interface between polyurethane sealants or caulks and a solid inorganic substrate, a polysulfide silane adhesion promoter, or its hydrolyzate and/or condensate, having the following formula:
(S~)a[R - SlX3]b wherein n is a num~er ranging from about 2 to about 6, a is a number equal to one-half the free valence of R, b is a number equal to at least 2, R is a divalent organic radical joined at one end to sulphur and at the other end to sillcon, and X is a hydrolyzable group The result ls a bond of the polyurethane seaLant or caulk to the inorganic substrate which has greater moisture resistance.

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The polyurethane sealants and caulks encompassed by this in~ention are the con~entional polyurethane sealant and caulking composi~ions known in ~he art.
Polyurethanes may be reacti~e insofar as they contain free lsocyanato groups or they may be wholly thermo-plastic reacted polymers essentiaLly ree of reackive isocyanate groups. The polyurethanes may be termina~ed by hydroxyl or amino groups.
The caulks or sealants will typically contain the typical fillers, or, if desired, they may be used ~ree of illers.
The polyurethane polymers are typically formed by the reaction of an organic polyisocyanate and an org~nic polyol. I~pically an organic diisocyanate is reacted with an organic diol whereby to form a polymer which is either isocyanate endblocked by virtue of using a stoichiometric excess of diisocyanate or it may be hydroxyl terminated by virtue o using a stoichiometric excess o the diol I the polymer contains free isocyanato group, it is typically called a "prepolymer'1.
It may be further reacted, or chain extended, with another or the same diol or a polyamine.
Polyurethanes are classed as elastomers by virtue o their having limited crosslinking by allophanate or biuret linkages, or they are essentially linear and the elastom2r qualities are provided by the presence o soft and hard segments in the polymer backbone~

7~79 ': ,`
D-9827 ~ ~.

Suitable polyurethanes are the segmented polymers of soft, low-temperature melting hydroxyl-terminated polymers which have been bonded through urethane linkages to stiff, high-temperature melting urethane, polyamide, polyurea, and/or polyester polymers which have been terminated with isocyanato or groups reactable with polyisocyanates (such as hydroxyl, -~
amino, mercapto, and the like). .~
The more de~irable polyurethanes typically`~;
possess at least one,preferably at least two, recurring polyether radical, that is, a polymeric moiety possessing recurring ether linkages i.e., -C-O-C- wherein the carbon atoms adjacent the oxygen are saturated, in the internal chain structure thereof, and/or at least one,~`
preferably at least two, recurring polyester radical, ^
that is, a polymeric moiety possessing recurring es~er O - .
"
linkages, i.e., -C-O-, in the internal chain structure thereof. The polyether and polyester radicals preferably possess a molecular weight of at least about 500 and not in excess of about 7,000. They are joined to the re-mainder of the polymer by urethanyl linkages, i.e., O R
.:
-O-C-N-, wherein R may be hydrogen or an organic group such as alkyl of from 1 to about 8 carbon atoms, cyclo-alkyl of from 5 to 8 carbon atoms, phenyl, or benzyl.
The urethanyl linkage is bonded to a carbon atom of the ~7~79 ~` organlc residue of an organic diisocyanate which in turn is joined thro~gh the nitrogen atom of an amide linkage (i.e., -N-C-~ to one of the active hydrogen-free (as detenmined by the well known Zerewitinoff method) residue of, e.g., an organic diol, a polyamine compound or amino to form a urea linkage (.e. -NH-C-NH-). m e poiyether and polyester radicals as described herein and in the claims may also contain urethanyl linkages o~ the type deseribed a~ove in the chain thereo~. Such radicals de~irably ha~e a melting point below 150~., and preer-~bly below 60C.
The polyester radical may be ormed by the reaction of a dicarboxylic acid with an organic diol or by the condensation polymerization of an alpha-omega-hydroxy-carboxylic acid or an alpha-omPga-~actone.
Preerably, these polyesters are hydro~yl end-blocked in~that the end groups of the polyest~r are hydroxyl bonded to noncarbonyl containing carbon atoms. These polyesters are then reacted9 if they are of the desired molecular weight, with an organic diisocyanate, most desira~ly ln the ratio of at least 2 moles o~ diisocyanate to one mole of the polyester, to form a diisocyanato end-block pr~polymerO Thls prepolymer is then reacted with a chain extender such as diol or dithiol chain extenders, di~mino chain extenders, or water, to form a substantially ~9~07~

linear, sol~ent-soluble polyurethane. A process or the manufacture of the aforemen~ioned polyure~hanes are des-cribed in U.S. Patent 3,09791920 Specific illustrations of chain extenders include hydrazine, ethylene diamine, 1,3-propylene diamine, 1,4-butane diamine, 1,6-hexamethyl-ene diamine, 1,4-piperazine, ethylene glycol, 1,2-propylene glycol~ 1,4-butane diol, ethanol ~mine, diethanolamine, urea, dimethylol urea, and the llke.
Other suitable polyesters may be formed by the react~on of epsilon caprolacton~ and/or alkyl-subs~ituted epsilon caprolactone and an active hydrogen containing ~nitiator such as water, ethylene glycol~ ethylene diamlne9 diethylene glycol9 dipropylene glycol, or !
192-propylene glycol, such as des.cribed in U.S. Patents, ~os. 3,169,945, 3,1869971; and 3~427,346.
~he polyesters possessing hydroxyl e~d groups ~nd having a molecular weight in excess of 500 and up ~o 7~000 may then be reacted with an organic diisocyanate to produce a polyurethane prepolymer having a mole~ular weight of from about 1,000 up to about 10,000. This polyuret~an~ may be isocyanato end-bloc~ed for direc~ :
reaction with the chain extender or may be hydroxyl end-blocked and is considered a prepolymer for additional reactlon with diisocyanate, as described in U~S~ Patent No. 3,186 ~971r .

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Another polyurethane which is most suitably employed is that described in U.S. Pa~ent No. 2,871,218.
The polyester-polyurethane of this patent is made by admixing a hydroxyl end-blocked or terminated polyester, formed by the reaction of 1,4-butane diol with adipic acid, with diphenylmethane-p,pi-diisocyanate and 1,4-butane diol in essentially exact stoichiometric proportions. The polyester should have a molecular weight of about 800 to 1,200 and for every mole of poly-ester there is employed from about 1.1 to 3.1 moles othe diisocyanate and from about 0.1 to 2.1 moles of the butane diol. By inereasing the mole amount of ocyanate9 it is possible ~o increase the melting ~sln~ and hardness of the resulting polyurethane and ~y reducing the mole æmount of diisocyanate, it is possible to decrease the melting point and hardness of the resultlng polyurethane.
The polyethers may be characterized in essen-tially the same manner as the polyesters above. They fall in the same melting point ranges~ are desirably in the same molecular weight range and are hydroxyl end-blocked or terminated. They are formed by the alkaline or acid condensation of alkylene oxidesO Such poly-ethers and their utilization in polyurethanes are described in U.S. Patent Nos. 2~813,776; 2,818,404;
2~929,800; 2,929,803; 2,929j804; 2,9~8,707; 3~180,853 and RE 24,691.

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D-9827 ~
~.
A particularly desirable method for making -:
linear polyurethanes is described in U. S. Patent No. 3,915,923~
With respect to the polysulfide substituted silanes in which R is alkylene, their manu~acture is described by F. Thurn and S. Wolffg in ~heir presentation entitled "New Organo Functional Silanes or the Tire Industry" at ~he International Session of German Rubber Society in Munich on September 2-5~ 1974, and in German patents 2,141,159 and 2,141,160, both published on March 1~ 1973. When R is alkylene, a is ~- :
oneO In UOS Patent No. 4,044,037 there is described a variety of polysulfide substituted aryl silanes having the average formula R' :~ :

[X-3 y Si ~(R)n -Ar) ]a [~S)x]b -~

wherein X represents a hydrolyzable group such as halogen, alkoxy and acyloxy radicals; Rl represents an alkyl radical containing from 1 to 4 carbon atoms;
y has a value of from 0 to 2 inclusive; R represen s a divalent bridging group such as alkylene and alkyleneoxy radicals containing from 1 to 7 carbon atoms; n has a value of 0 to 1; Ar represents an aryl radical containing, e.g., ~rom ~C17~47~ .

6 to 12 carbon atoms; (S)x represents a divalent poly- :
sulfide radical each free valence of which is directly bonded to an aromatic carbon atom of an Ar radical whereby each Ar radical is bonded to another Ar radical through a (S)x radical; x ha6 a value of from 2 to 6; a has a value of at least 2; b has a value of at least 1; and the ratio of a to b is a value of not more than 2.
Specific illustratives of silanes which are covered by this invention are those which are charactPrized by the following formula:
:

3 iCH2CH2CH2 S S -CH2CH2CH2six3 3 2 2CH2 S S S ~ CH2CH2CH2siX3 X3siCH2CH2CH2-- S S--S --S CH2CH2CH2SiX3 X SiCH CH-CH2 - S ~ S ~ S CH2l 2 3 :
X3sicH~cH2cH2cH2-s-s-s-s-c:H2cH2cH2cH2six3 ., X3SiCH2--S ~ S--S--S--SCH2SiX3, and mixture of the above;

.. . . .. . . . . .. ..

!
~si~ 5 ~35i~

~Si~-~S - 5~5 s-5)3--l~35iX3 cl ~-si-c2H~ (S~ ~c~H4-si~cl3 :
C:H3 ~H3 ~13 ~(5)2 ~1-(5~2 ~3 C~H4-S1- C13 C~H4~ Si- C13 ~3 CH3 C~3 ~ 3 ~(5)2 ~(5)2 -~(5) 2-~

~14-Si-C13 { 2~4-si-c13 C 21H4-s~ 3 C2~ Si-C13 lq~71479 D-9827 [13-5i-C2U4 ~ 2 [ 2.8 wherein Ar is a tolylene radical )3 Si C2~14-lr¦2 ¦(5)2 wherein Ar is a tolylene radical [2U503-Si-C2H4-A~ 2 ~ )2.8 wherein Ar is a tolylene radical ~C13 si c~u4-Ar ] 2 ~ S~2 6 wherein Ar is a tolylene radical ~130)3-Si-C2U4-~2 ~S)261 ~

wherein Ax i~ a tolylene radical : . . . .

D 4~27 ~07~47g _ _ ~ _ :~

C13-Si-C2H~-A~ 2 LS)3.63 j wherein Ar is phenylene ~CH30)3 si C2H4 12 [ )3 wherein Ar is a phenylene ~ 1 -Si-C H4- ~ ~5) 25 wherein Ar is a pheny1ene r lr (C~I30)3 Si-C~H4-Ar ( ~3.25 ~ _ 2 wherein Ar is a phenylene radical -Si-C2H4- ~ 2 [ ~2.90 wherein Ar is a phenylene ~H30~3-51-C2H4-~2 ~5)2.90 wherein Ar is a phenylene ~07 1 4~9 D-9827 LCH30~3-Si-C2~4- ~ 2 ~ )3'7 wherein Ar is a phenylene ~ 3 5 2H4 ~ 2 [S~2.36¦

; where;n Ar is a tolylene 2H40)3 Si C2H4 ~ 2 ~5)2.3 whe~ein Ar is a tolylene . The amount of ~he silane adhesion promoter which is incorporated into a sPalant or caulklng formula-tion, a technique called l'integral blending'l, may range ~rom as little as 0~01 weight % to about 6 weight V/o~
based on the total weight of the composition, pre~erably from about 1 weight % to about S weight % based on the total weight of the sealant or caulkin~ composition.
~ he æmount of silane which is used in p~inling a su~strate before coating the sealant or caulk to the su~strate is ~hat amount of silane which is capable of en~ancing the adhesion of the caulk or sealant to the ~ubstrate. ~o ~mount is regarded to be crl.tical. How-~er~ one should attempt to avold putting down too thick a layer of silc~ne to a substrate slnce th~s will cause ~ ~5 -~7~79 D~9827 the sealant or caulk to form a weaker adhesion boundary layer where it contac~s the substrate or where the silane coatlng contacts ~he sealant or caulk. Usually, a solvent solution of the silane is applied ko the substrate followed by evaporation of the solvent to leave behind a substrate which is primed with the silane.
It is important to appreciate that the ~mount of the silane w~îch -is used regardless of whether the technique for adhesion promotion is integral blending or priming should be sufficient to enhance the adhe~ion of the sealant or caulk to the su~strate.
It should be recognized that the surfaces of the substrate should be properly cleaned prior ~o applying either the primer or the sealant or caulk containing the silane adhesion promoker.
A typical polyurethane formula is illustrated by the following:
Part 1 Parts by Wei~t Hydroxyl-terminated polyester resin (MultronTM R-68, Mobay Chemical Co.~ Pittsburgh, Pa.) 96.2 ~ethylene dianiline 1.6 Isocyanate-terminated prepolymer** 2t2 TiO~ 16 Carbon Black 0.16 Dibutyltirldilaurate 0.16 Molecular Sieve 2.5 Silane Adheslon Promoter 1.0 *Damusis, "Sealants", Re-inhold Publ. Corp., ~.Y , 1967, page 146.

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Part 2 Par~s by Weight ** See above -Isocyanate~terminated prepolymer (MondurTM MR, Mobay Chemlcal Company, Pittsburgh~ Pa.~ 10.9 Various additives for the controL of properties are used which include extender pigments as mica, clay, talC~ alwmlna, sla~e dusk, etc. Other compounding ingredi-ents include dried or calcined pigments, shrinkage minimiz-ing materials such as aluminum powder and zirconium silicate, fire retardants as phosphorous triisocyanate P(NCO)3 or phosphoryl triisocyanate 0-P(NCo)3, and bodying agents as organic or inorganic ib~rs. NaturalLy, th2 above silanes are added as adhesion promoters9 and the usual ~ntioxidants (IonolTM, ZalbaTM, e~c.) and ultra-violet absorbers (UvionolTM 400 or Cryosor ~ W-24) are required in many recipes.
The ollowing i5 a typlcal procedure* for making a polyurethane single package sealant system:
Mix in a resin kettle equipped with stirr~r and distillation head; under inert atmosphere:

A. 53.8 parts of polypropylenP ether diol, molecular weight about 39000 36.0 parts o inely ground CaCO3 18.2 parts of finely ground Ti02 B. 63.7 parts of polypropylene ether triol, molecular weight about 4,000 C. 138.0 parts Gf pe~roLeum naphtha.
A~ter mixing, heat to distill o 50 parts of petroleum naphtha and allow to cool to 70~C.
To t~e above mixture add:

*See Chu et al., U.S. 3,711,445, patent Jan. 16, 1973, at col. 4, line 20 et sequence.

~7~7~ D 9827 D, 45.9 parts of bis (4-isoc~anatophenyl) methane and a drop ~,3 cc,) of stannous octoate catalyst, Stir ~or 10 minutes, Heat to 100-105C, for one hour and cool to room temperature, Add:
E, 67,0 parts of naphthenic and paraffinic process oil blend 37,5 parts of fumed silica, ollowed by one hour of stirring, to produce a urethane sealant, Silane ~oupling agents are preferably added before the isocyanate, if desired, In the above, all parts are parts by weight, The Eollowing methods were used or cleaning the test panels used in the experiments below:
1, 'rhe aluminum panels were degreased with methylethylketone solvent and allowed to dry, 2, Each aluminum panel was thoroughly rubbed with 00 grade steel wool, to remove the oxide layer and were washed with a water solut.ion of an AlconoxTM
laboratory detergent abrasive cleaner until it was com-pletely wetted by a film of water which after draining, left a film o water on the surface, 3, The panels were rinsed in distilled water~
blotted with paper towels and allowed to dry in a horizontal position at room temperature for several hours, Similarly, glass test panels were cleaned as follows:
1, The glass was scrubbed with AjaxTM cleanser on a cloth pad until dipping in~water and draining showed a flow-off of the water as a film rather than droplets, ~ 18 -~ ~ 7 ~ ~7 2. The panels were rinsed in distilled water~
blotted wlth clean paper toweling, and aLlowed to dry in a horizontal posLtion at room temperature in a 50% rela-tive humidity atmosphere or several hours.
These preparation proeedures exceed the require-ments of Federal Specification TT-S000227E for sealing compounds.
The method employed or the testing and evalua-tion of all silane compounds in this example is described below:
The silane primer materials wexe 5 weight %
30~ution of the silane adhesion promoters A and C, cited below, in a 90/10 mixture by weight of SD-3A (denatured) ethanol/water and a 5 weight % solution o~ silane adhesion promoter, B and D~ cited below, in toluene. Ihey werP
applied to suitably cleaned and dried (as described above~
- glas~ and a1umin~m paneLs as test su~strates, allowing several hours to dry at room temperature.
The silane prLmer solutions were applied on the panels by brushing with a clean camels hair brush which had been dipped lnto the primer solution and lightly drained by touching the rim o~ the storage bottle con-tainer. The primed panels were then placed flat on a benchtop and allowed to dry ~or 3 or 4 hours at room temperature Two ~ommercially available urethane sealant compositions were used ln these evaluations ~hey were 10 7 1 ~7 9 D-9827 single-package, cartridge type seal~nts, obtained directly from the manufacturer. Two different colors o sealant, white and tan, were used The sealant was applied to the test panels a;s beads of sealant extruded from ~he standard hand-held type caulking gun to provide uniform~
smooth9 3/8 inch diameter beads of material running the ~ull length of the 6 inch long test panels.
The sealant was allowed to dry overnight at room temperature at about 50% relative humidity9 followed by thre~ weeks exposure to 100% relative humidity at room temperature (~20-25~C.). The adhesion was examined after this initial three-week cure, then the test panels were ~mmersed in tap water and tested after 7 days immersion, and again after 30 days immersion in the tap water at room temperature.
The adhesion o~ the cured sealant beads to the test panels was measured by partially stripping the cured sealant from the substrate and noting whether failure was adhesive at the substrate surface, indi~ating poor adhesion, or cohesive within the sealant, indicating the bond to the substrate was desirably stronger than the cohesive forces within the cured sealant.
Ex~erimental:
Aluminum and glass test panels were primed with the 5% solutions of the silanes listed below, and allowed to dry for several hours. Beads o~ white and tan commer-cial polyurethane-based sealants were applied to the 1071~79 D-9827 primed panels. After curing three weeks at room tempera-ture, 100% relative humidity, there were no s~gns of adhesive failure, only cohesive failure. Similarlyl after one week and four weeks and six months water Lmmer-sion periods, the peel tests showed only co~esive failure indicating excellent adhesion of the sealant to the primed test panels.
The same commercially available sealants similarly applied to cleaned unprimed glass and aluminum su~strates (test panels) showed complete adhes~ve failure in that the sealant beads were easily stripped from the unprimed glass and aluminum surfaces.
SILANES TESTED
Example A bis-(gamma-trimethoxysilylpropyl) disulfide ~ bis-(gamma-trimethoxysilylpropyl) trisulfide C bis-(gamma-trimethoxysilylpropyl) tetrasulfide D bls-(4-tr~methoxysilyltolyl) trlsulfide

Claims (10)

WHAT IS CLAIMED IS:

1. The process of enhancing the bonding of polyurethane sealants and caulk compositions to solid inorganic surfaces which comprises providing at the interface of the caulk or sealant composition and said surface a silane, or the hydrolyzate or condensate of silane, having the formula:

(Sn)a[R - SiX3]b wherein n is a number ranging from about 2 to about 6, a is a number equal to one-half the free valence of R, b is a number equal to at least 2, R is a divalent organic radical joined at one end to sulphur and at the other end to silicon, and X is a hydrolyzable group.

2. The process of claim 1 wherein R in the silane is alkylene.

3. The process of claim 1 wherein the silane has the formula:

[(S)x]b, wherein X represents a hydrolyzable group; R' represents an alkyl radical containing from 1 to 4 carbon atoms; y has a value of from 0 to 2 inclusive;
R° represents a divalent bridging group containing from 1 to 7 carbon atoms; n has a value of 0 to l; Ar represent an aryl radical containing from 6 to 12 carbon atoms; (S)x represents a divalent polysulfide radical each free valence of which is directly bonded to an aromatic carbon atom of an Ar radical through a (S)x radical; x has a value of from 2 to 6; a' has a value of at least 2; b' has a value of not more than 2.

4. The process is claim 2 wherein the silane is bis-(gamma-trimethoxysilylpropyl) disulfide.

5. The process of claim 2 wherein the silane is bis-(gamma-trimethoxysilylpropyl) trisulfide.

6. The process of claim 2 wherein the silane is bis-(gamma-trimethoxysilylpropyl) tetrasulfide.

7. The process of claim 3 wherein the silane is bis-(4-trimethoxysilyltolyl) trisulfide.

8. A solid inorganic surface having bonded thereto a polyurethane sealant or caulk composition wherein the bond is enhanced by the presence at the interface of said caulk or sealant composition and said inorganic surface of a silane, or the hydrolyzate or condensate of a silane, having the formula:
(Sn)a[R -SiX3]b wherein n is a number ranging from about 2 to about 6, a is a number equal to one-half the free valence of R, b is a number equal to at least 2, R is a divalent organic radical joined at one end to sulphur and at the other end to silcon, and X is a hydrolyzable group.

9. The product of claim 8 wherein R in the silane is alkylene.

10. The product of claim 8 wherein the silane has the formula:

wherein X represents a hydrolyzable group; R' represents an alkyl radical containing from 1 to 4 carbon atoms;
y has a value of from O to 2 inclusive; R° represents a divalent bridging group containing from 1 to 7 carbon atoms; n has a value of 0 to l; Ar represents an aryl radical containing from 6 to 12 carbon atoms;
(S)x represents a divalent polysulfide radical each free valance of which is directly bonded to an aromatic carbon atom of an Ar radical whereby each Ar radical is bonded to another Ar radical through a (S)x radical;
x has a value of from 2 to 6; a' has a value of at least 2; b' has a value of at least 1; and the ratio of a' to b' is a value of not more than 2.