s i l ic o n e c he m i s try
Source: wichientep - Fotolia.com
48
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.
O
rganosilane 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
e u r o p e an co at i ngs J OURNAL 03 – 2016
(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: hydroly-
sis 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-
s i li c o ne chem i s t ry
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-
49
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.
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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
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50
s i l ic o n e c he m i s try
 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 allow-
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.
e u r o p e an co at i ngs J OURNAL 03 – 2016
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 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
s i li c o ne chem i s t ry
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 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 approximately 25 °C and 50% relative humidity.
Degree 1 of the DIN 53 150 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
51
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 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 
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52
s i l ic o n e c he m i s try
 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 127149.
[2]
Torry S.A. et al, Kinetic analysis of organosilane 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/
LexUriServ/LexUriServ.do?uri=OJ:L:2010:08
6:0007:0012:EN:PDF
[4]Safeguards SGS Consumer Testing Services,
No. 062/10 April 2010, http://newsletter.
sgs.com/eNewsletterPro/uploadedimages/000006/SGS-Safeguards-06210-Commission-Publishes-Regulation-AmendingREACH-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
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 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. F abrics 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.
≤ 0.1%
1 January 2015
Dioctyltin (DOT) compounds
1. Textile articles intended to come into contact with the skin,
2. Gloves,
3. F
oot 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
(RTV-2 moulding kits).
≤ 0.1%
1 January 2012
e u r o p e an co at i ngs J OURNAL 03 – 2016
s i li c o n e chem i s t ry
53
Table 2: Degree of dryness (DIN 53 150).
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
DMS polymer [10]
TMS
DES
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
100.0
100.0
100.0
Total
Table 4: Degree of dryness (hours).
Catalyst /
drying degree
1
2
3
4
5
6
7
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
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
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
DMS polymer
TMS polymer
DES polymer
e u r op e an coat i ngs JOURNA L 03 – 2016
54
s i l ic o n e c he m i s try
“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.
Dr Ravi
Ravichandran
Vice-President, Research &
Development
King Industries
[email protected]
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.
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, http://www.ecosimpflooring.com/download/adesivi-silanici-origine-diffusione_eng.pdf
[7] “Model 415” Drying Time Tester, Erichsen GmbH & Co. KG.
Table 5: Mechanical properties (cure: 2 weeks ambient except
where noted otherwise).
Shore A
Stress at
max psi
Strain at
max %
Modulus
psi
0.6% DOTDAA
52
378
234
265
2.0% organometallic
52
324
256
243
0.6% DOTDAA
52
313
320
189
2.0% organometallic
52
359
362
173
0.5% DOTDAA*
22
93
267
85
2.0% organometallic
48
291
608
103
DMS polymer
TMS polymer
DES polymer
*1 month ambient cure.
e u r o p e an co at i ngs J OURNAL 03 – 2016
[8]Instron Corporation, Shore A durometer.
[9]Instron Corporation, Dual column table top model, 30 kN (6700 lbf) load
capacity.
[10]M S Polymer “S303H” dimethoxymethylsilane polymer, Kaneka Corporation, Osaka, JP.
[11]M S Polymer “SAX520”, trimethoxysilane polymer, Kaneka Corporation,
Osaka, JP.
[12] Diethoxysilane polymer, Easterly Research, Warminster, PA.
s i li c o ne chem i s t ry
55
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