non-tin catalysts for alkoxy silane polymers€¦ · non-tin catalysts for alkoxy silane polymers...

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NoN-tiN catalysts for alkoxy silaNe polymers

regulatory restrictions on the use of tin catalysts create considerable difficulties for formulators of crosslinkable silane-terminated polymer systems. a tin-free organometallic catalyst can give good mechanical properties with similar cure times, and even cure one system where tin compounds are ineffective.

o rganosilane polymers are used in ad-hesives, sealants and coatings as either

coupling agents or crosslinkers. As coupling agents, their role is to promote adhesion between organic and inorganic substrates. As crosslinkers, organosilane polymers react with other functional groups to form covalent bonds that can generate products with struc-tural properties.Crosslinkable organosilane polymers consist of functionalised backbones with alkoxysilane terminal groups. The main silane terminated polymer backbone chemistries are polyether

organometallic catalysts developed for moisture cured silane-terminated polymer systems. By John Florio, ravi ravichandran, David switala and Bing hsieh, King industr ies, inc.

(Figure 1), polysiloxane and polyurethane. Cured properties such as Tg and flexibility are dependent on the backbone chemistry. Organosilane polymers can promote adhe-sion, weatherability and reinforcement of coatings, adhesives, sealants and fillers. Or-ganosilanes are monomeric compounds with at least one silicon atom bonded to a carbon. Siloxane compounds (-Si-O-Si-) are polymeric organosilane compounds, and silanol groups (-Si-OH) are hydrolysed silane groups.Since their development in the late 1970s, modified silane polymer products have be-come widely used in DIY and construction seal-ants and adhesives for a variety of indoor and outdoor applications on a variety of substrates.

orgaNosilaNe crossliNkiNg iN-volves two reactioNs

The fundamental organosilane crosslinking process involves two key reactions: hydroly-

sis of an alkoxysilane functional group to generate silanol groups (Figure 2) and con-densation of the silanol group with other functional groups. The silanol groups will undergo a condensation reaction with other silanol groups (Figure 3) or with alkoxysilane groups (Figure 4), each producing crosslinked siloxane bonds.The condensation of two silanol groups will generate water and, as shown in Figures 2 and 4, hydrolysis and condensation of alkox-ysilane groups will generate an alcohol by-product.For the purpose of this discussion, it is con-venient to assume that the hydrolysis reac-tion occurs initially, followed by the con-densation reactions. However, in practice hydrolysis and condensation occur concur-rently unless special efforts are made to sep-arate the steps [1].Alkoxysilane polymers are used in single-com-ponent moisture cure applications. For exam-

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Results at a glance

ű A new tin-free organometallic compound has been developed for the catalysis of crosslinkable silane-terminated polymer systems. The catalyst can accelerate the reaction of alkoxy-silane terminated resins based on polyether and polyure-thane backbone chemistries.

ű It can provide a range of me-chanical properties for various caulk, sealant and adhesive appli-cations while providing cure times similar to tin catalysed systems.

ű In the specific case of diethoxy-silane polymers, which have the advantage of not emitting toxic methanol during cure, tin is a very poor catalyst but the organometallic product tested produced an effec-tive cured product within a reason-able cure time.

extensively studied and tin compounds sug-gested as being the most active. However, the tin catalysts must initially hydrolyse to form the active species [2].Tin compounds are commonly used to cata-lyse the crosslinking of alkoxysilane systems, particularly systems based on methoxysilane polymers. Compounds that efficiently cata-lyse many of these crosslinking reactions in-clude dioctyltin diacetyl acetonate and dibu-tyltin dilaurate.However, concerns about toxicity of tin com-pounds have driven formulators to explore other catalyst options. Although these op-

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ple, formulated alkoxysilane products can be stored in sealed cartridges for caulk and sealant applications. Commercial moisture cure caulks and sealants based on alkoxysilanes are typically advertised to have a shelf life of nine months.The product becomes activated when it is applied to a substrate and exposed to atmospheric moisture. Since the cure involves two concurrent reaction pro-cesses, accelerating the overall curing reaction will require a catalyst that con-tributes to both processes.There are several variables that play key roles in determining the reaction rates of moisture cure alkoxysilane systems. The hydrolysis and condensation reaction rates are dependent on the pH of the system and on the alkyl substituents on the silicon [1]. The other key component in determining the overall reaction rate is the catalyst.

tiN aNd other catalysts: curreNt situatioN summarised

The hydrolysis and condensation reac-tions can be accelerated by acids, bases and organometallics. Specific mecha-nisms have been described for each of the acid and base catalysed reactions [1]. The activity of organometallic com-pounds in organosilane systems was

tions can include acids, bases and other organometallics, finding a catalyst that can provide sufficient reaction acceleration has been elusive.Along with reaction acceleration, physical properties of the cured product can also be dependent on the catalyst. For example, some acids might provide good accelera-tion of the crosslinking reaction, but the acid might also accelerate rearrangement of the formed polysiloxane backbone, causing prod-uct degradation. Some tertiary amines could accelerate the crosslinking reaction, but they might also contribute colour and odour.

s i l i c o n e c h e m i s t r y5 0


Figure 3: Formation of siloxane crosslinks by condensation of silanols.

Figure 4: Formation of siloxane crosslinks by condensation of silanol and alkoxysilane.

Figure 2: silanol formation, hydrolysis of alkoxysilanes.

Figure 1: structure of alkoxysilane polymer.

implicatioNs of eu regulatioNs limitiNg tiN coNteNt

European Commission Decision 2009/425/EC, which includes restrictions on the use of dibutyltin, dioctyltin and tri-substituted or-ganotin compounds, was incorporated into ANNEX XVII of REACH through regulation (EU) 276/2010 [3]. A summary of these restric-tions is in Table 1 [4].With the stigma of being environmentally regu-lated, tin compounds are more often avoided, regardless of the dosage required to sufficient-ly accelerate the reaction. For example, suffi-cient acceleration of the reaction of polyols with polyisocyanates for many 2-component coating applications usually requires levels of tin metal that are well below the ≤ 0.1% limi-tation. Coatings formulators still often strive to formulate completely tin-free systems. The level of tin metal required to achieve sufficient cure of moisture cured organosilane polymer

systems is typically very close to the ≤ 0.1% limit established in REACH Annex XVII, Entry 20. Therefore, while tin replacement is an is-sue for the polyurethane coatings industry, it is a greater issue for industries that use moisture cured organosilane polymer coatings, adhe-sives and sealants.

methaNol levels iN curiNg may exceed safety limits

If the R groups in Figures 2 and 4 are methyl, then the by-product generated in the hydrol-ysis and condensation reactions would be methanol. The European Agency for Safety and Health at Work (EU-OSHA) directive 67/548/EEC, and the 25th updating of this directive (98/98/EC), have defined methanol as harmful with danger of very serious irreversible effects by inhalation, skin contact and ingestion.According to Commission Directive 2006/15/EC of February 7, 2006, the maximum allow-

able methanol exposure for an eight-hour workday is 260 mg/m3. Methanol exposure studies of a methoxysilane floor adhesive based on a NIOSH method [5] have been conducted that report 5400 mg/m3 emission of methanol during an eight hour period [6].An approach to completely eliminating meth-anol from the alkoxysilane curing process is to use ethoxylated silane polymers. However, catalysis of the ethoxysilane crosslinking re-action is challenging. Tin compounds have proved to be inefficient for these reactions.

orgaNometallic catalyst is studied iN three systems

The main motivation for replacing tin in mois-ture curing organosilane systems is that of regulatory issues, not performance issues. An extensive study was conducted to identify compounds that could be considered as alter-natives. Included in the study were a range of



metal compounds, acids, amines, acid/amine salts and combinations of each.From this list, one catalyst provided perfor-mance comparable to tin catalysts in dimethoxy and trimethoxy silane systems and superior performance in a diethoxysilane system.This catalyst, referred to as “K-KAT 670”, is a non-tin organometallic compound designed to provide activity that is comparable to tin catalysts for crosslinking moisture cure organosilane sys-tems. This catalyst can potentially accelerate the curing process by all three of the catalytic mech-anisms reviewed and will not accelerate rear-rangement of the polysiloxane backbone. This product was compared to tin catalysts in sev-eral different formulated alkoxysilane systems. Experiment I is based on a dimethoxymethylsilyl (DMS) polyether polymer and Experiment II on a triethoxysilyl (TMS) polyether polymer system. Experiment III addresses the issue of methanol generation by using an organosilane based on a diethoxysilyl (DES) polyether polymer.

productioN aNd test procedures

Fully formulated single component moisture cure alkoxysilane systems were used in the

experiments. The uncatalysed formulations were stored in dispensing cartridges. Approx-imately 30 grams of uncatalysed material was dispensed into a container with a caulk gun before addition of the catalyst.The material in the container was mixed on a “SpeedMixer” rotary mixer for 30 seconds at 1500 rpm then 2 minutes at 2200 rpm. An adjustable doctor blade was used to apply 3 mm thickness of the blend onto a paper sub-strate. The degree of dryness was determined by using a Model 415 Drying Time Tester [7] in accordance with DIN 53 150.The dryness test involved applying a force onto a paper disk that covered a test area on the casting for 60 seconds. The results are based on the amount of tack and on visual impressions that develop from the applied force. The dryness testing was done at ap-proximately 25 °C and 50% relative humidity. Degree 1 of the DIN 53 150 method was sub-stituted by a glove test to determine touch dry. Table 2 defines the rating system used.Hardness of the castings was determined with a Shore A [8] hardness tester after the castings were allowed to cure under ambient conditions for two weeks. Other mechanical

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properties were measured on an Instron [9] tester using dog-bone shaped samples cut from the fully cured 3 mm thick castings.

performaNce matched oN dimethoxymethylsilaNe…

The organometallic catalyst was compared to dioctyltin diacetylacetonate (DOTDAA) in a fully formulated system based on a polyether backbone DMS polymer. Catalyst levels were derived from ladder studies. Levels of orga-nometallic were adjusted to produce cast-ings that dried at rates similar to the system catalysed with 0.6% DOTDAA.The tin content of DOTDAA is approximately 21%. At 0.6%, the tin content in the formu-lated control system is approximately 0.12%, which would not comply with EU regulations. The general formulation is in Table 3.Dryness ratings of 3 mm thick castings ac-cording to DIN 53 150 are in Table 4. The systems dried similarly, with each achieving the highest degree of dryness (paper does not adhere to 20 kg load, no visible change to coated surface) in six hours.Differences in the performance of the two



table 1: summary of organotin requirements under european regulation (eU) 276/2010 amending Annex XVii of reAch, reAch Annex XVii, entry no 20.

substance scope requirement Date effective

tri-substituted organostannic compounds such as tributyltin (tBt) compounds, triph-enyltin (tpt) compounds and Dibutyltin (DBt) compounds

article or part of an article ≤ 0.1% 1 July 2010

1. Mixture 2. article or part of an article (except food contact materials)

≤ 0.1% 1 January 2012

Dioctyltin (Dot) compounds

1. one-component and t wo-component room temperature vul-canisation sealants (rt V-1 and rt V-2 sealants) and adhe-sives,

2. paints and coatings containing DBt compounds as catalysts when applied on articles,

3. soft polyvinyl chloride (pVc) profiles whether by themselves or coextruded with hard pVc,

4. Fabrics coated with pVc-containing DBt compounds as stabi-lisers when intended for outdoor applications,

5. outdoor rainwater pipes, gutters and fittings, as well as cover-ing material for roofing and facades.

≤ 0.1% 1 January 2015

Dioctyltin (Dot) compounds

1. textile articles intended to come into contact with the skin, 2. gloves, 3. Foot wear or part of foot wear intended to come into contact with

the skin,4. Wall and floor coverings,5. childcare articles,6. Female hygiene products,7. nappies,8. two-component room temperature vulcanisation moulding kits

(rt V-2 moulding kits).

≤ 0.1% 1 January 2012

systems were not significant based on the mechanical property results (Table 5). The two castings developed similar tensile stress (which can be associated with toughness), modulus (elastic modulus) and strain (elongation).

…almost equalled oN trimethox-ysilaNe…

DOTDAA was also compared to the organo-metallic catalyst in a polyether backbone TMS polymer system. The general uncatalysed formulation is in Table 3. The dryness ratings in Table 4 indicate slightly higher reactivity with the DOTDAA catalysed system.The DOTDAA system achieved the maximum dryness rating (passed 20 kg load test) in five hours while the non-tin catalyst required six hours to reach the 7 dryness rating. The cast-ings developed similar mechanical properties after the two weeks of ambient cure (Table 5).

…while tiN performs poorly iN diethoxysilaNe polymer

Results of the DES polymer study were very different compared to the DMS and TMS re-sults. The basic uncatalysed DES formulation is in Table 3. DOTDAA was essentially not ac-tive in this system at 0.5% and 1.0% on total formulation weight.Higher dosages were not evaluated since the tin level incorporated with the 1.0% dosage

was more than double the maximum allowed by EU regulations. The 3 mm thick castings required more than 120 hours to achieve a dryness rating of 7. Dryness ratings are in Ta-ble 4 and mechanical properties in Table 5.To investigate a tin compound with a different ligand, dibutyltin dilaurate (DBTDL) was added to the study. However, as with DOTDAA, activ-ity of the DBTDL system was poor. Dry times of the DES system with organometallic catalyst were significantly faster than the tin catalysed systems and indeed were comparable to the DMS and TMS systems. The system also com-pleted five weeks of storage at 50 °C with no loss of activity.The DOTDAA system required a month of am-bient cure before it was suitable for testing on the Instron. Even so, the casting had very weak properties.

regulatory compliaNce aNd a broader applicatioN area

Addressing tin regulatory restrictions, new tin-free organometallic compounds have been developed that have demonstrated activity similar to tin catalysts for accelerat-ing moisture cure methoxysilane crosslinking reactions. The test catalyst demonstrated activity that was similar to tin in dimethoxy-methylsilane and trimethoxysilane polymer systems.Also addressed in this work was the catalysis

of a non-methanol-emitting moisture cure system based on a diethoxysilane polymer. In this case, the non-tin product catalysed the crosslinking reaction while tin catalysts were essentially not active at double the maximum tin concentration levels allowable under cur-rent EU regulations.

refereNces

[1] Osterholtz F.D., Pohl E.R., Kinetics of the hydrolysis and condensation of organofunc-tional alkoxysilanes: a review, Jnl. adhesion sci. technol., 1992, Vol. 6, no. 1, pp 127-149.

[2] Torry S.A. et al, Kinetic analysis of organosi-lane hydrolysis and condensation, internat. Jnl. adhesion & adhesives, 2006, Vol. 26, pp 40-49.

[3] commission regulation (eu) no 276/2010 of 31 March 2010, http://eur-lex.europa.eu/Lexuriser v/Lexuriser v.do?uri=oJ:L:2010:086:0007:0012:en:pDF

[4] safeguards sgs consumer testing ser vices, no. 062/10 april 2010, http://newsletter.sgs.com/enewsletterpro/uploadedimag-es/000006/sgs-safeguards-06210-com-mission-publishes-regulation-amending-reacH-restrictions-en-10.pdf

[5] niosH method (national institute for occupa-tional safety and Health, usa)- method 2000, issue 3.



table 2: Degree of dryness (Din 53 150).

table 3: experimental system formulations.

1 touch dry, no visible residue remaining on rubber glove

2 paper does not adhere, but visible change with 20 g load

3 paper does not adhere, but visible change with 200 g load

4 paper does not adhere, but visible change to coated surface with 2 Kg load

5 paper does not adhere, no visible change to coated surface with 2 Kg load

6 paper does not adhere, but visible change to coated surface with 20 Kg load

7 paper does not adhere, no visible change to coated surface with 20 Kg load

component Dms tms Des

DMs polymer [10] 32.8

tMs polymer [11] 32.8

Des polymer [12] 20.2

plasticiser 16.4 16.4 23.1

Filler 39.3 39.3 49.1

titanium dioxide 6.6 6.6 3.3

thixotrope 1.6 1.6 --

antioxidant -- -- 0.3

HaLs 0.3 0.3 0.3

uVa 0.3 0.3 0.8

Moisture scavenger 0.7 0.7 1.4

adhesion promoter 2.0 2.0 1.5

total 100.0 100.0 100.0

table 4: Degree of dryness (hours).

catalyst / drying degree

1 2 3 4 5 6 7

DMs polymer

0.6% DotDaa 0.8 2.3 3.5 4.0 4.5 5.0 5.5

2.0% organometallic 1.5 2.3 3.0 3.5 4.5 5.5 6.0

tMs polymer

0.6% DotDaa 0.3 0.5 1.0 1.3 3.0 3.3 5.0

2.0% organometallic 1.0 1.5 2.0 2.3 4.0 4.3 6.0

Des polymer

0.5% DotDaa 120+ 120+ 120+ 120+ 120+ 120+ 120+

2.0% organometallic 1.3 3.0 4.5 5.0 6.0 6.5 7.0



“Addition of the tin-f ree compounds does not require extraordinary

incorporation techniques.“

Dr ravi ravichandranV ice-president, research & DevelopmentK ing industr iesr avichandr an@kingindustr ies.com

3 questions to Dr ravi ravichandran

In short, why is tin seen as critical? While tin is viewed as a versatile catalyst, there are consid-erable regulatory and toxicological hurdles on the horizon, that is encouraging users to phase out these catalysts.The changes in classification and possible labelling changes to account for repro-ductive and mutagenic toxicity, have led formulators to strive for tin free sytems where possible. In addition REACH has established limits and restrictions on the use levels of these catalysts, in various applications.

How elaborate is it to incorporate the tin-free compounds into existing formulations? Ad-dition of the tin-free compounds does not require extraordinary incorporation techniques. Depend-ing on the system, they are preferably added to the formulation after steps that generate or require high temperatures.

Are further property improvements feasible with these compounds? Along with the regulato-ry benefits of being tin-free, an important benefit of using the K-KAT 670 is its activity in ethoxysilane systems, which do not produce methanol byproduct and are poorly catalysed with tin compounds. In addition they provide a range of mechanical properties for various caulk, sealant and adhesive applications while providing cure times similar to tin catalysed systems.

table 5: mechanical properties (cure: 2 weeks ambient except where noted otherwise).

shore A stress at max psi

strain at max %

modulus psi

DMs polymer

0.6% DotDaa 52 378 234 265

2.0% organometallic 52 324 256 243

tMs polymer

0.6% DotDaa 52 313 320 189


Des polymer

0.5% DotDaa* 22 93 267 85


*1 month ambient cure.

[6] Galbiati Dr. A., Maestroni Dr. F., silane adhesives: origin, diffusion and environmental problems, n.p.t. research unit, gropello cairoli, pavia, italy, 2008, http://www.ecosimpflooring.com/download/adesivi-silani-ci-origine-diffusione_eng.pdf

[7] “Model 415” Drying time tester, erichsen gmbH & co. Kg.

[8] instron corporation, shore a durometer.[9] instron corporation, Dual column table top model, 30 kn (6700 lbf) load

capacity.[10] Ms polymer “s303H” dimethoxymethylsilane polymer, Kaneka corpora-

tion, osaka, Jp.[11] Ms polymer “saX520”, trimethoxysilane polymer, Kaneka corporation,

osaka, Jp.[12] Diethoxysilane polymer, easterly research, Warminster, pa.



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non-tin catalysts for alkoxy silane polymers€¦ · non-tin catalysts for alkoxy silane polymers...

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