Non-tin catalysts for alkoxy silane polymers

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48 silicone chemistry Source: wichientep - Fotolia.com Non-tin catalysts for alkoxy silane polymers Organometallic catalysts developed for moisture cured silane-terminated polymer systems.

1 48 silicone chemistry Source: wichientep - Fotolia.com Non-tin catalysts for alkoxy silane polymers Organometallic catalysts developed for moisture cured silane-terminated polymer systems. By John Florio, Ravi Ravichandran, David Switala and Bing Hsieh, King Industries, Inc. Regulatory restrictions on the use of tin catalysts create considerable difficulties for formulators of crosslinkable silaneterminated 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. Organosilane polymers are used in adhesives, 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 structural properties. Crosslinkable organosilane polymers consist of functionalised backbones with alkoxysilane terminal groups. The main silane terminated polymer backbone chemistries are polyether (Figure 1), polysiloxane and polyurethane. Cured properties such as Tg and flexibility are dependent on the backbone chemistry. Organosilane polymers can promote adhesion, weatherability and reinforcement of coatings, adhesives, sealants and fillers. Organosilanes 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 become widely used in DIY and construction sealants and adhesives for a variety of indoor and outdoor applications on a variety of substrates. Organosilane crosslinking involves two reactions The fundamental organosilane crosslinking process involves two key reactions: hydrolysis of an alkoxysilane functional group to generate silanol groups (Figure 2) and condensation 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 alkoxysilane groups will generate an alcohol byproduct. For the purpose of this discussion, it is convenient to assume that the hydrolysis reaction occurs initially, followed by the condensation reactions. However, in practice hydrolysis and condensation occur concurrently unless special efforts are made to separate the steps [1]. Alkoxysilane polymers are used in single-component moisture cure applications. For exam-

silicone chemistry 49 Results at a glance űű A new tin-free organometallic compound has been developed for the catalysis of crosslinkable silaneterminated polymer systems.

2 silicone chemistry 49 Results at a glance űű A new tin-free organometallic compound has been developed for the catalysis of crosslinkable silaneterminated polymer systems. The catalyst can accelerate the reaction of alkoxy-silane terminated resins based on polyether and polyurethane backbone chemistries. űű It can provide a range of mechanical properties for various caulk, sealant and adhesive applications while providing cure times similar to tin catalysed systems. extensively studied and tin compounds suggested as being the most active. However, the tin catalysts must initially hydrolyse to form the active species [2]. Tin compounds are commonly used to catalyse the crosslinking of alkoxysilane systems, particularly systems based on methoxysilane polymers. Compounds that efficiently catalyse many of these crosslinking reactions include dioctyltin diacetyl acetonate and dibutyltin dilaurate. However, concerns about toxicity of tin compounds have driven formulators to explore other catalyst options. Although these op- 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 acceleration of the crosslinking reaction, but the acid might also accelerate rearrangement of the formed polysiloxane backbone, causing product degradation. Some tertiary amines could accelerate the crosslinking reaction, but they might also contribute colour and odour. űű In the specific case of diethoxysilane 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 effective cured product within a reasonable cure time. 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 processes, accelerating the overall curing reaction will require a catalyst that contributes 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. Innovative solutions through Technical Progress Binders and Additives for the Paint and Coatings Industry Modern production facilities and management systems Customer-oriented R&D-Team with its own application technology Constantly growing range of products made from renewable raw materials High-quality commercial products from well-known partners Market experience since 1851 Tin and other catalysts: current situation summarised The hydrolysis and condensation reactions can be accelerated by acids, bases and organometallics. Specific mechanisms have been described for each of the acid and base catalysed reactions [1]. The activity of organometallic compounds in organosilane systems was Visit us at the Eurocoat 2016 at booth B 60. Worlée-Chemie GmbH Söllerstraße D Lauenburg european coatings 0 service@worlee.de JOURNAL

50 silicone chemistry Implications of EU regulations limiting tin content European Commission Decision 2009/425/ EC, which includes restrictions on the use of dibutyltin, dioctyltin and

3 50 silicone chemistry 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 organotin compounds, was incorporated into ANNEX XVII of REACH through regulation (EU) 276/2010 [3]. A summary of these restrictions is in Table 1 [4]. With the stigma of being environmentally regulated, tin compounds are more often avoided, regardless of the dosage required to sufficiently accelerate the reaction. For example, sufficient 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% limitation. 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 issue for the polyurethane coatings industry, it is a greater issue for industries that use moisture cured organosilane polymer coatings, adhesives 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 hydrolysis 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 allowable methanol exposure for an eight-hour workday is 260 mg/m 3. 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 methanol from the alkoxysilane curing process is to use ethoxylated silane polymers. However, catalysis of the ethoxysilane crosslinking reaction 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 moisture curing organosilane systems is that of regulatory issues, not performance issues. An extensive study was conducted to identify compounds that could be considered as alternatives. Included in the study were a range of Figure 1: Structure of alkoxysilane polymer. Figure 2: Silanol formation, hydrolysis of alkoxysilanes. Figure 3: Formation of siloxane crosslinks by condensation of silanols. Figure 4: Formation of siloxane crosslinks by condensation of silanol and alkoxysilane.

silicone chemistry 51 metal compounds, acids, amines, acid/amine salts and combinations of each.

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 systems.

This product was compared to tin catalysts in several different formulated alkoxysilane systems.

Experiment III addresses the issue of methanol generation by using an organosilane based on a diethoxysilyl (DES) polyether polymer.

Approximately 30 grams of uncatalysed material was dispensed into a container with a caulk gun before addition of the catalyst.

An adjustable doctor blade was used to apply 3 mm thickness of the blend onto a paper substrate.

4 silicone chemistry 51 metal compounds, acids, amines, acid/amine salts and combinations of each. From this list, one catalyst provided performance 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 systems. This catalyst can potentially accelerate the curing process by all three of the catalytic mechanisms reviewed and will not accelerate rearrangement of the polysiloxane backbone. This product was compared to tin catalysts in several 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. Approximately 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 substrate. The degree of dryness was determined by using a Model 415 Drying Time Tester [7] in accordance with DIN 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 approximately 25 C and 50% relative humidity. Degree 1 of the DIN method was substituted 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 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 organometallic were adjusted to produce castings 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 formulated 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 according to DIN 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 The Colorants Company Colorants & Pigment dispersions Your new partner!

5 52 silicone chemistry 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 trimethoxysilane DOTDAA was also compared to the organometallic 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 castings 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 results. The basic uncatalysed DES formulation is in Table 3. DOTDAA was essentially not active 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 Table 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, activity 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 completed five weeks of storage at 50 C with no loss of activity. The DOTDAA system required a month of ambient 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 accelerating moisture cure methoxysilane crosslinking reactions. The test catalyst demonstrated activity that was similar to tin in dimethoxymethylsilane 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 current EU regulations. REFERENCES [1] Osterholtz F.D., Pohl E.R., Kinetics of the hydrolysis and condensation of organofunctional alkoxysilanes: a review, Jnl. Adhesion Sci. Technol., 1992, Vol. 6, No. 1, pp [2] Torry S.A. et al, Kinetic analysis of organosilane hydrolysis and condensation, Internat. Jnl. Adhesion & Adhesives, 2006, Vol. 26, pp [3] Commission Regulation (EU) No 276/2010 of 31 March 2010, LexUriServ/LexUriServ.do?uri=OJ:L:2010:08 6:0007:0012:EN:PDF [4] safeguards SGS Consumer Testing Services, No. 062/10 April 2010, sgs.com/enewsletterpro/uploadedimages/000006/sgs-safeguards commission-publishes-regulation-amending- REACH-Restrictions-EN-10.pdf [5] niosh method (National Institute for Occupational Safety and Health, USA)- method 2000, Issue 3. 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, Triphenyltin (TPT) compounds and Dibutyltin (DBT) compounds Dioctyltin (DOT) compounds Dioctyltin (DOT) compounds Article or part of an article 0.1% 1 July Mixture 2. Article or part of an article (except food contact materials) 1. one-component and two-component room temperature vulcanisation sealants (RTV-1 and RTV-2 sealants) and adhesives, 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 stabilisers when intended for outdoor applications, 5. outdoor rainwater pipes, gutters and fittings, as well as covering material for roofing and facades. 1. Textile articles intended to come into contact with the skin, 2. Gloves, 3. Footwear or part of footwear 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 (RTV-2 moulding kits). 0.1% 1 January % 1 January % 1 January 2012

silicone chemistry 53 Table 2: Degree of dryness (DIN 53 150).

6 silicone chemistry 53 Table 2: Degree of dryness (DIN ). 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 Table 3: Experimental system formulations. Component DMS TMS DES DMS polymer [10] 32.8 TMS polymer [11] 32.8 DES polymer [12] 20.2 Plasticiser Filler Titanium dioxide Thixotrope Antioxidant HALS UVA Moisture scavenger Adhesion promoter Total Table 4: Degree of dryness (hours). Catalyst / drying degree DMS polymer % DOTDAA % organometallic TMS polymer % DOTDAA % organometallic DES polymer 0.5% DOTDAA % organometallic

54 silicone chemistry Addition of the tin-free compounds does not require extraordinary incorporation techniques. 3 questions to Dr Ravi Ravichandran In short, why is tin seen as critical?

7 54 silicone chemistry Addition of the tin-free compounds does not require extraordinary incorporation techniques. 3 questions to Dr Ravi Ravichandran In short, why is tin seen as critical? While tin is viewed as a versatile catalyst, there are considerable 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 reproductive 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? Addition of the tin-free compounds does not require extraordinary incorporation techniques. Depending on the system, they are preferably added to the formulation after steps that generate or require high temperatures. Dr Ravi Ravichandran Vice-President, Research & Development King Industries ravichandran@kingindustries.com Are further property improvements feasible with these compounds? Along with the regulatory 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. [6] Galbiati Dr. A., Maestroni Dr. F., Silane adhesives: origin, diffusion and environmental problems, N.P.T. Research Unit, Gropello Cairoli, Pavia, Italy, 2008, [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 Corporation, Osaka, JP. [11] MS Polymer SAX520, trimethoxysilane polymer, Kaneka Corporation, Osaka, JP. [12] Diethoxysilane polymer, Easterly Research, Warminster, PA. 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 % organometallic TMS polymer 0.6% DOTDAA % organometallic DES polymer 0.5% DOTDAA* % organometallic *1 month ambient cure.

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New types of silicone resin open up wider fields of application. By Marco Heuer, Evonik Resource Efficiency GmbH.

50 SILICONE RESINS Source: by-studio - Fotolia.com TAKING THE HEAT New types of silicone resin open up wider fields of application. By Marco Heuer, Evonik Resource Efficiency GmbH. The properties of the