Non-Tin
Catalysts
46
| FEBRUARY 2016
f0r
Cr0sslinkable
Silane
Terminated
P0lymers
JOHN FLORIO AND RAVI RAVICHANDRAN, KING INDUSTRIES, INC., USA
rosslinkable organosilane systems can be
based on a variety of chemistries and can be utilized for a
variety of applications. Selecting a proper
catalyst is a critical step in the formulating
process of organosilane systems. Along with
the increasing need to replace tin catalysts
for regulatory reasons, some tin catalyzed
organosilane systems lack activity and
performance properties. A new series of tinfree compounds has been developed for the
catalysis of crosslinkable silaneterminated polymer systems. Each of the
developed catalysts demonstrates superior
activity with different organosilane chemistries used for a variety of applications.
Si
OH
CH3O
Si
Introduction
Organosilane polymers are utilized in
adhesives, sealants, and coatings as either
coupling agents or crosslinkers. As coupling agents, their role would be 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.
Catalysis of organosilane crosslinking
reactions is a system-specific process,
particularly if the objective is to use a
tin-free catalyst. Organosilane crosslinking chemistries include one-component
moisture-cured systems that can be based
Si
O
Si
CH3OH
Alkoxy
C2H5
Si
OH
C2H5
C
NO
Si
Si
O
Si
CH3
C
NOH
CH3
Oxime
O
Si
OH
O
CH3CO
Si
Si
O
Si
CH2COH
Si
O
Si
(CH3)2NH
Acetoxy
Si
OH
(CH3)2N
Si
Amine
CH2
Si
!
!
OH
CH3CO
O
Si
Si
O
Si
CH3CCH3
Enoxy
This article first reviews the crosslinking process of one-component alkoxysilane-terminated systems,
oneand two-component
PDMS systems
crosslinked with oximinosilane, and two-component PDMS
FIGURE
1—Common
silanol condensation
reactions.
systems crosslinked with acetoxysilane. This review is followed by an in-depth summary of different
catalyst processes for organosilane chemistries. Finally, a catalyst evaluation for each of the chemistries
is presented to demonstrate the effect different catalyst chemistries have on each system.
!!
!
Organosilane Crosslinking Processes A Head
A basic structure for an alkoxysilane polymer is shown in Figure 2. Complete crosslinking of a onecomponent moisture cure alkoxysilane system requires acceleration of two chemical processes. In this PAINT.ORG
case, the catalyst must accelerate both the hydrolysis of the alkoxysilane (Figure 3) and condensation of
the formed silanol groups (Figure 4), or condensation of other alkoxysilane groups with formed silanol
|
47
ndensation reactions. However, in practice, hydrolysis and condensation occur concurrently unless
ecial efforts are made to separate the steps.1
condensation reactions. However, in practice, hydrolysis and condensation occur concurrently unless
are made
separateasthe
steps.1component in sealed tubes and applied
oducts basedspecial
on thisefforts
chemistry
can betosupplied
a single
th dispensers
for caulk, sealant,
andHowever,
adhesiveinapplications.
They areand
widely
used for DIY
projects
and
condensation
reactions.
practice, hydrolysis
condensation
occur
concurrently
unless
Products
based
on
this
chemistry
can
be
supplied
as applications
a single component
in sealed
tubes and applied
1outdoor
mmercial construction
projects
for atovariety
of indoor
and
on a variety
of
special efforts
are made
separate
the steps.
condensation
reactions.
However,
in
practice,
hydrolysis
and
condensation
occur
concurrently
with
dispensers
for
caulk,
sealant,
and
adhesive
applications.
They
are
widely
used
for
DIY
projects
bstrates. Organosilane polymers can promote adhesion, weatherability,
and reinforcement of coatings, unless and
1
special
efforts
are
to projects
separate
the
commercial
construction
for
a steps.
variety
of as
indoor
and component
outdoor applications
a variety
of
hesives, sealants
and
fillers.
Products
based
onmade
this chemistry
can
be
supplied
a single
in sealed on
tubes
and applied
substrates.
Organosilane
polymers
promote
adhesion, weatherability,
and
reinforcement
of coatings,
with
dispensers
for caulk, sealant,
andcan
adhesive
applications.
They are widely
used
for DIY projects
and
Products
based
on
this
chemistry
can
be
supplied
as
a
single
component
in
sealed
tubes
and
applied
adhesives,
sealants
and
fillers.
GURE 2—Structure
ofconstruction
alkoxysilane
polymer.
commercial
projects
for a variety of indoor and outdoor applications on a variety of
with
dispensers
for
caulk,
sealant,
and
adhesive
applications.
They
are
widely
used
for
DIY
projects
and
substrates. Organosilane
polymers can
promote
adhesion, weatherability,
and reinforcement of coatings,
CH3
CH3
CH
3
commercial
construction
projects
for a variety
of indoor and outdoor applications
on a variety of
FIGURE 2—Structure
of alkoxysilane
polymer.
adhesives,
sealants
and fillers.
substrates. Organosilane polymers
can
promote
adhesion,
weatherability,
and
reinforcement
of
coatings,
CH3
CH3
CH3
RO
Si
O
CHCH2O
Si
OR
adhesives,
sealants
and
FIGURE
2—Structure
offillers.
alkoxysilane polymer.
n
RO CH
Si3
O CH
CHCH
Si3
OR
CH
2O
3
FIGURE
2—Structure
of alkoxysilane
polymer.
OR
OR
!
n
CH3
CH3
CH3
RO
SiOR
O
CHCH2O
SiOR OR
!
FIGURE
2—Structure
ofSialkoxysilane
polymer.
GURE 3—Silanol
formation,
hydrolysis
of alkoxysilanes.
RO
O
CHCH2O n
Si
OR
OR
OR
!
CH3
CH
CH3 n
3
FIGURE 3—Silanol formation,
hydrolysis of alkoxysilanes.
OR
OR
!
O
O
O
H CH
HCH
H CH
!
!
!
!
!!
!!
!
!
!!
2
3
2
2
3
3
P Si OR
P Si OHhydrolysis of alkoxysilanes.
P Si OH
2ROH
FIGURE 3—Silanol formation,
Catalyst
Catalyst
Catalyst
H 2O
H 2O
H 2O
PCH
Si
P CH
Si3 of
OHalkoxysilanes.
P CH
Si3 OH
3 OR
FIGURE
3—Silanol
formation, hydrolysis
OR
OR
OH
Catalyst
Catalyst
Catalyst
CH3
CH3
CH3
H 2O
H 2O
H 2O
OR
P SiOR
OH
P SiOH
OH
! P SiOR
H 2O
H 2O
H 2O
Catalyst
Catalyst
Catalyst
P Si OR
OH
GURE 4—Formation
of siloxane crosslinksPby Si
condensation
of silanols. P Si OH
Catalyst
Catalyst
Catalyst
OR
OR
OH
!
CH
CH
CH
CH
!
!
!
!
!
!
!
2ROH
2ROH
2ROH
3 FIGURE
3
3 silanols.
3 of siloxane
FIGURE 3—Silanol
formation,
hydrolysis
of alkoxysilanes.
4—Formation
crosslinks
by condensation
of
OR
OR
OH
!
Catalyst
CH3
CH3
CH3
CH3
P Si FIGURE
OH
P Si O Si
P
4—Formation
siloxane crosslinks by condensation
of silanols.
P Si ofOH
+
H2O
Catalyst
FIGURE
4—Formation
of siloxane
crosslinks
by condensation of
silanols.
3 OH
P CH
Si3 OH
P CH
Si3 O CH
Si3 P
P CH
Si
+
H2O
OR
OR CH3
OR
CH3
CH3
CH3
CatalystOR
P SiOROH
P SiORO SiORP
P SiOROH
+
H2O
Catalyst
!
P
Si
OH
P
Si
O
Si
P
P
Si
OH
+
H2O
GURE 5—Formation of siloxane crosslinks by condensation of silanol and alkoxysilane.
OR
OR
OR
OR
!
CH3 FIGURE
CH
CH
CH
3
3
3
5—Formation of siloxane crosslinks by condensation of silanol and alkoxysilane.
OR
OR
OR
OR
!
Catalyst
CH3
CH3
CH3
CH
3
FIGURE
4—Formation
ofsiloxane
siloxane
crosslinks
by Pcondensation
ofsilanol
silanols.
P Si FIGURE
OH
Si O Siof
P
5—Formation
crosslinks by condensation
and
P Si ofOR
+ alkoxysilane.
ROH
Catalyst
CH
3 P
FIGURE
5—Formation
of siloxane
by condensation P
of CH
silanol
3 OR
P CH
Si3 OH
Si3 Oand
Sialkoxysilane.
P CH
Sicrosslinks
+
ROH
OR
OR CH3
OR
CH3
CH3
CH3
CatalystOR
P SiOROH
P SiORO SiORP
P SiOROR
+
ROH
Catalyst
! catalysts
ficiency of non-tin
for accelerating the crosslinking
of one- and two-component silanolSi OH
P Si
O Si P
P SiExamples
OR of crosslinkers for one+
ROH
rminated PDMS P
systems
can vary significantly.
and two-component
catalysts for
accelerating
the crosslinking
oneand two-component
ORof
OR
ORshown in silanolOR
anol-terminated
PDMS
arenon-tin
acetoxysilanes
and
oximinosilanes.
Crosslinkingofmechanisms
are
! Efficiency
terminated
PDMS systems
can
significantly.
Examples
of the
crosslinkers
for one- and two-component
gures 6 and 7.
The crosslinking
processes
forvary
these
chemistries
differ from
one-component
OR
OR
OR
OR
!
silanol-terminated
PDMS are acetoxysilanes
and oximinosilanes. Crosslinking
mechanisms are shown in
!!
!!
!
!
!!
!
oisture-cured
systemsofwith
alkoxysilane
groups in
that
they do not
moisture
to initiate silanolEfficiency
non-tin
catalyststerminal
for accelerating
the
crosslinking
ofrequire
one- and
two-component
Figures 6PDMS
and 7.systems
The crosslinking
forExamples
these chemistries
differ from
the one-component
e reaction. terminated
can vary processes
significantly.
of crosslinkers
for oneand two-component
Efficiency
of
non-tin
catalysts
for
accelerating
the
crosslinking
of
oneand
two-component
silanolmoisture-cured
systems
with
alkoxysilane
terminal
groups
in
that
they
do
not
require
moisture
to initiate
silanol-terminated
PDMS of
aresiloxane
acetoxysilanes
and by
oximinosilanes.
Crosslinking
mechanisms
are shown
in
FIGURE
5—Formation
crosslinks
condensation
of silanol
and
alkoxysilane.
terminated
PDMS
systems
canoximinosilane
vary
significantly.
Examples
of crosslinkers
for
oneand
the
reaction.
ith the silanol
groups
already
formed,
the
andthese
acetoxysilane
crosslinked
systems
are two-component
less
Figures
6 and
7. The
crosslinking
processes
for
chemistries
differ from
the
one-component
silanol-terminated
PDMS
are
acetoxysilanes
and
oximinosilanes.
Crosslinking
mechanisms
are
shown
able than the
alkoxysilane systems
atmospheric
to that
generate
reactive
silanolmoisture to initiate in
moisture-cured
systems that
withrequire
alkoxysilane
terminalmoisture
groups in
they do
not require
Figures
6 and
7.
The
crosslinking
processes
these
chemistries
differ
from the
one-component
With
the
silanol
groups
already
formed,
the for
oximinosilane
acetoxysilane
crosslinked
systems are less
oups. However,
even
with
the
presence
of silanol
functional
groups,
theand
systems
containing
the
the
reaction.
withthan
alkoxysilane
terminal
groups
in thatand
they
do not
require moisture
initiate
stable thanare
thesystems
alkoxysilane
systems
require alkoxysilane
atmospheric
moisture
to generate
reactive to
silanol
iminosilanemoisture-cured
crosslinker
more
stable
thosethat
containing
acetoxysilane
second,
less
utilized
approach
involves the
on
different
functionalized
backbone
the
reaction.
groups.
However,
even
with
the
presence
ofasilanol
groups,
osslinkers. With
The
carbonyl
group
in
the
acetoxy
groupthe
has
higher functional
activation
effect onthe
thesystems
Si–OC containing
the silanol
groups
already
formed,
oximinosilane
and acetoxysilane
crosslinked
systemsthe
are less
oximinosilane
crosslinker
are
more
stable
than
those
containing
alkoxysilane
and
acetoxysilane
use of toangenerate
organic
peroxide
polymers
that havesystems
alkoxysilane
stable
than the alkoxysilane
that require termiatmospheric moisture
reactive
silanol in a radicalWith
theHowever,
silanolThe
groups
already
formed,
and
acetoxysilane
crosslinked
systems
crosslinkers.
carbonyl
group
in thethe
acetoxy
has
agroups,
higher
activation
effect
on the
Si–OC
groups.
even
the
presence
ofoximinosilane
silanolgroup
functional
the systems
containing
the are less
induced
polymerization.
nalthan
groups.
Thewith
backbone
chemistries
stable
thecrosslinker
alkoxysilane
that
require
atmospheric
to generate
reactive!3silanol
oximinosilane
aresystems
more stable
than those
containingmoisture
alkoxysilane
and acetoxysilane
groups.
However,
even
with
the
presence
of
silanol
functional
groups,
the
systems
containing
the polydimethyl
crosslinkers.
carbonyl could
group inbe
the polyether,
acetoxy group has a higherHydroxy-terminated
activation effect on the Si–OC
of theseThe
systems
oximinosilane crosslinker are more stable than those containing alkoxysilane and acetoxysilane
!3
crosslinkers.
The carbonyl
group in the acetoxy
group has a higher
activation are
effectcurable
on the Si–OC
siloxanes
to elastomers
polysiloxane,
or polyurethane.
Cured
!
!
!
properties (e.g., Tg, hardness, flexibility,
etc.) can be dependent on the chemistry
of the backbone. Other cure chemistries involve one- and two-component
systems crosslinking silanol-terminated
polydimethoxysilane (PDMS) with
alkoxysilane, oximinosilane, or acetoxysilane crosslinkers.
In addition to the condensation reactions which are the focus of this article,
vinyl functional silanes are commonly
cured by an addition reaction which
involves the addition of a silicon hydride
to a terminal vinyl doublebond. Platinumbased homogeneous strong acids like
chloroplatinic acid (Karstedt acid) are typically used here and the cures can either
be at ambient or higher temperatures. A
48
| FEBRUARY 2016
!3
by a wide variety of crosslinkers.
!3
Equations listing the most common
cure pathways are presented in Figure
1. All of these systems undergo condensation cure resulting in elimination of a volatile byproduct such
as acetic acid, an oxime, or alcohol.
Metallic corrosion can result from
elimination of acetic acid or possibly
alcohols. Oxime or enoxy cure systems
are preferred for applications that
involve metal contact. The previously
mentioned homogeneous platinum
catalyzed systems are utilized in many
applications that involve electronics
and fiber optics because of issues with
volatile byproducts or corrosion associated with silanol-based systems.
This article first reviews the crosslinking process of one-component
alkoxysilane-terminated systems, oneand two-component PDMS systems
crosslinked with oximinosilane, and
two-component PDMS systems crosslinked with acetoxysilane. This review
is followed by an in-depth summary of
different catalyst processes for organosilane chemistries. Finally, a catalyst
evaluation for each of the chemistries
is presented to demonstrate the effect
different catalyst chemistries have on
each system.
Organosilane
Crosslinking Processes
A basic structure for an alkoxysilane
polymer is shown in Figure 2. Complete
crosslinking of a one-component moisture cure alkoxysilane system requires
acceleration of two chemical processes.
In this case, the catalyst must accelerate both the hydrolysis of the alkoxysilane (Figure 3) and condensation of
the formed silanol groups (Figure 4),
or condensation of other alkoxysilane
groups with formed silanol groups
(Figure 5) to produce crosslinked
siloxane structures. As demonstrated in
Figures 3 and 5, 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 initially occurs, followed by
the condensation reactions. However,
in practice, hydrolysis and condensation occur concurrently unless special
efforts are made to separate the steps.1
Products based on this chemistry can
be supplied as a single component in
sealed tubes and applied with dispensers for caulk, sealant, and adhesive
applications. They are widely used for
DIY projects and commercial construction projects for a variety of indoor and
outdoor applications on a variety of
substrates. Organosilane polymers can
promote adhesion, weatherability, and
reinforcement of coatings, adhesives,
sealants and fillers.
©ISTOCK / HALFPOINT
Efficiency of non-tin catalysts for
accelerating the crosslinking of one- and
two-component silanol-terminated PDMS
systems can vary significantly. Examples
of crosslinkers for one- and two-component
silanol-terminated PDMS are acetoxysilanes and oximinosilanes. Crosslinking
mechanisms are shown in Figures 6 and
7. The crosslinking processes for these
chemistries differ from the one-component
moisture-cured systems with alkoxysilane
terminal groups in that they do not require
moisture to initiate the reaction.
With the silanol groups already formed,
the oximinosilane and acetoxysilane
crosslinked systems are less stable than
the alkoxysilane systems that require
atmospheric moisture to generate reactive
silanol groups. However, even with the
presence of silanol functional groups, the
systems containing the oximinosilane
crosslinker are more stable than those
containing alkoxysilane and acetoxysilane crosslinkers. The carbonyl group in
the acetoxy group has a higher activation
effect on the Si–OC hydrolysis, compared
to the C=N double bond in the oxime
group.2 Formulated oximinosilane systems
can sometimes be supplied as a onecomponent system because the oximinosilane is more resistant to hydrolysis.
Catalysis of Organosilane
Crosslinking
The hydrolysis and condensation reaction rates of organosilane systems are
dependent on pH of the system and on
the substituents on the silicon. The rate
minimum for the hydrolysis and condensation reactions occurs at approximately pH 7 and pH 4, respectively.
Each pH change of one unit in either
direction produces a 10-fold rate acceleration, assuming an excess of water
is available. At pH > 10, hydrolysis of
the first intermediate, RSi(OR)2OH, is
inhibited due to the ionization of the
silanol group. 3 The effect of pH on the
organosilane hydrolysis and condensation reaction rates is illustrated in
Figure 8. Both hydrolysis and condensation reactions are reversible. Alcohols
will reverse the silane hydrolysis.
The hydrolysis and condensation reactions can be accelerated by
acids, bases, and organometallics.
Osterholtz and Pohl described specific mechanisms for each of the acidand base-catalyzed reactions. Torry,
Campbell, Cunliffe and Tod extensively
studied activity of organometallic
compounds in organosilane systems
and they propose tin compounds as the
most active. However, the tin catalysts
must initially hydrolyze to form the
active species.4 Tin compounds are
commonly used to catalyze the crosslinking of organosilane systems including alkoxy, acetoxy, and oximinosilane
systems. Dioctyltin and dibutyltin compounds that efficiently catalyze many
of these crosslinking reactions include
dioctyltin diacetylacetonate and dibutyltin dilaurate. However, concerns
about toxicity of tin compounds have
driven formulators to explore other catalyst options. Although the options can
include acids, bases, and other organometallics, finding a single non-tin
catalyst that could provide sufficient
reaction acceleration for all of the
mentioned silane crosslinking chemistries is difficult. Along with reaction
acceleration, physical properties of the
cured product can 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
color and odor.
PAINT.ORG |
49
hydrolysis, compared to the C=N double bond in the oxime group.2 Formulated oximinosilane systems
can sometimes be supplied as a one-component system because the oximinosilane is more resistant to
hydrolysis.
!
FIGURE 6—Silanol-terminated PDMS reaction with oximinosilane.
CH3
HO
Si
CH3
O
Si
N
N
O
N
O
N
Si
O
OH
n
CH3
Base-Catalyzed Condensation
of Silanols
O
CH3
Catalyst
!
!
Polymeric Siloxane
!
N
OH
!! !!
FIGURE 6—Silanol-terminated PDMS reaction with oximinosilane.
FIGURE
PDMS
reaction
with acetoxysilane.
7.
Silanol7—Silanol-terminated
Terminated PDMS Reaction
with
Acetoxysilane
O
O
O
CH3
HO
CH3CH
CH3
3
CH3
OH
n
n
CH3CH
3
O
O
Si
SiHO OSi SiO OH
CH3
O
Si
SiO
O
O
O
O
O
O
OO
O
Catalyst
Catalyst
O
!!
!
Polymeric
Siloxane
Polymeric
Siloxane
!
!
O
OH
OH
7—Silanol-terminated
PDMSAreaction
Catalysis ofFIGURE
Organosilane
Crosslinking
Head with acetoxysilane
revised copy (replace everything from Experiment V head to Acknowledgments. Note new
also included:
The hydrolysis and condensation reaction rates of organosilanecharge
systems
are dependenton
onthe
pH silicon
of the in the
development
system
on the substituents
on the silicon.
Thewith
rate acetoxysilane
minimum
for thecrosslinker
hydrolysis
and condensation
transition
state (T.S.1
or T.S.2) is considment
V. and
2-component
silanol terminated
PDMS
eactions
occurs atwas
approximately
pH 7aand
pH clear
4, respectively.
Eachon
pH
change
of one unit in either
wing experiment
conducted
with
simple
system based
a
silanol
terminated
erable.
This
isavailable.
supported
by >experimenOsterholtz
andrate
Pohl
proposed
a bimolecdirection
produces
a 10-fold
acceleration,
assuming
excess
ofof
water
is
At pH
10,
18 with
19 The an
an acetoxysilane
crosslinker.
weight
ratio
the resins
was 10:1.
ethoxysiloxane
3
is
inhibited
due
to
the
ionization
of
the
silanol
hydrolysis
of
the
first
intermediate,
RSi(OR)
tal
observations
where
electron-withular
nucleophilic
displacement
2OH,reaction
ere cast with a dry thickness of about 1.5 mils. Because the films were relatively thin, dry timesgroup.
The
effectwith
ofconsistent
pH
on thedrying
organosilane
hydrolysis
andas
condensation
reaction
rates
is illustrated
in alkyl
Figuregroups
8.
20
drawing
substitution
onpattern
the
withtime
their
kinetic
data,
ermined
circular
recorders.
This
method
is based
on the
circular
scribe
Both
hydrolysis
condensation
reactions
are reversible.
Alcohols
will reverse
hydrolysis.
oating
made by and
a teflon
sphere stylus
that makes
a complete
360° revolution
in 6the
h. silane
The three
!
Base-Catalyzed Hydrolysis
of Alkoxysilanes
depicted in Figure 9 for base catalysis
attached to the Si accelerates the rates
dry time stages are Set to Touch—the time when material stops flowing back into the channel
hydrolysis.
This
is due
of Surface
alkoxysilane
hydrolysis,
involving
by the stylus;
Dry—when
the stylus
no longer leaves aofclear
channel and
begins
to to the stabilization ofDry—when
the developing
negative
charge
a pentacoordinate
intermediate,
and
he dry upper
layer of the film (also
considered dust-free);
and Through
the stylus
no
!4
uptures or dents
the film. Alltransition
films were aged
at T.S.1
25°C and 50% which
RH.
lowers the energy of the transitwo different
states,
tion
states.
T.S.2.
the
a system
base and
em withoutand
catalyst
wasIn
still
wetpresence
after 6 h. of
The
catalyzed
with
0.05% DBTDL reached the
In General
water,
formed
ouch stage in
aboutthe
0.1 transition
h, a Surfacestate
Dry inT.S.1
aboutis0.25
h, and a Through
Dry in Base
aboutCatalysis
0.5 h. Eachof the
on-tin catalysts
included
in the
previouscharge
studieson
demonstrated
some
activity of
in this
study.
hydrolysis
alkoxysilanes,
any basic
with
a partial
negative
the
r, some of the catalysts developed compatibility issues that were evident in the formulated blends
species
accelerates
the
reaction
by
alkoxysilane
silicon
during
the
first
step
t and in the films. Table 11 includes results of systems catalyzed with the catalysts tested in
assisting
thezinc
removal
of a proton from
of the
hydrolysis
T.S.1
thenK-KAT XK-648,
ents I through
IV. An
addition toreaction.
the catalyst
list was
another
complex
The catalyst
dosages were
2% basedthe
on formula
weight.
water in the transition state. In Specific
dissociates
to generate
pentacoordinate intermediate which subsequently
Base Catalysis of the hydrolysis of
AT 670 catalyzed system developed relatively slow dry times and appeared to have a
alkoxysilanes,
theDBTDL
hydroxide anion
down
to the with
desired
silanol catalyst was
bility issue. breaks
The system
catalyzed
the proprietary
slower than the
particularly through
in SurfaceaDry.
It also
appeared to
be incompatible.
K-KAT XK-651
therate
best by directly
accelerates
theprovided
reaction
second
transition
state,
T.S.2.
ouch and Surface Dry of the non-tin catalysts, but Through Dry tailed off, possibly because of poor
attacking
the
substrate.
In
the
proposed
mechanism,
negative
bility that was evident in the film appearance. The K-KAT XK-648 catalyzed system developed
y times and it appeared to be compatible.
50
| FEBRUARY 2016
11—Experiment V, Circular Recorder Dry Times ()
Chojnowoski and Chrzczonowicz found
that secondary and tertiary amines
(piperidine, triethyl- amine, tri-nbutylamine) catalyzed the condensation
of a series of dialkyl- or diarylsilanediols in aqueous dioxane. 5 They proposed
that primary and secondary amines
catalyzed the reactions by nucleophilic
attack on silicon which was followed
by the rapid attack of dialkylsilanediol
on the silamine formed, as shown in
Figure 10. Tertiary amine catalyzed
reactions were proceeded by a general
base-catalyzed mechanism.
Acid-Catalyzed Hydrolysis
of Alkoxysilanes
The mechanism for the acid-catalyzed
alkoxysilane hydrolysis reaction proposed by Osterholtz and Pohl is shown
in Figure 11. According to Osterholtz and
Pohl, the mechanism for acid-catalyzed
hydrolysis is a rapid equilibrium protonation of the substrate, followed by a
bimolecular SN2-type displacement of the
leaving group by water.
Acid-Catalyzed Condensation
of Silanols
The acid-catalyzed condensation
reaction proceeds by an initial protonation of the silanol followed by an SN2
displacement reaction at the Si leading
to formation of water and regeneration of the acid catalyst as depicted in
Figure 12.
Tin Catalysis Mechanism
As previously mentioned, tin catalysts
are utilized in alkoxy, acetoxy, and
oximinosilane systems. The tin catalyst can be in the form of dioctyltin
and dibutyltin diacetylacetonate and
dibutyltin dilaurate (DBTDL).
Catalysis of organosilanes with tin
compounds begins with hydrolysis of
the tin compound. As shown in Figure
13, the tin compound undergoes hydrolysis and forms an organotin hydroxide,
which is the true catalytic species.6
The organotin hydroxide reacts with
Organosilane Hydrolysis and Condensa2on Effect of pH on Reac2on Rates an alkoxysilane group to form an
organotin silanolate (Figure 14). The
tin silanolate will react readily with
alcohols and water to form the silanol
(Figure 15). The organosilanolate will
also react with formed silanol groups
to produce siloxane linkages, and the
organotin hydroxide catalyst is regenerated (Figure 16).
-­‐1 Methanol Emission
If the R groups in Figures 2 and 4 are
methyl, then the byproduct 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,
P
Si
and the 25th updating of this directive
(98/98/EC), has defined methanol as
OH
!
harmful with danger of very serious
!!
!
1 2 3 4 5 6 7 8 9 10 11 log pK -­‐2 12 13 14 pH>10 hydrolysis
inhibited due to
ionization of
SiOH group
-­‐3 -­‐4 transition state (T.S.1 or T.S.2) is considerable. This is supported by experimental observations where
Hydrolysis
Condensation
electron-withdrawing
substitution on the alkyl groups attached
to the Si accelerates the rates of
minimum
rate
~pH 7
~pH 4 of the developing
hydrolysis. This isminimum
due to therate
stabilization
negative
charge
which lowers the energy of
the transition states.
Tin EU Regulations
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) 276/2010.7 A summary of these
restrictions is presented in Table 1.8
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 twocomponent coating applications usually
requires levels of tin metal that are well
below the ≤ 0.1% limitation. Regardless,
coatings formulators still often strive to
formulate completely tin-free systems.
The dosage of tin metal that is often
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, sealants.
0 !
-­‐5 pH In General Base Catalysis of the hydrolysis of alkoxysilanes,
any basic species accelerates the reaction
by assisting the removal of a proton from water in the transition state. In Specific Base Catalysis of the
Condensa4on hydrolysis of alkoxysilanes, the hydroxide
anion acceleratesHydrolysis the reaction rate by directly attacking the
substrate.
!
FIGURE 8—Organosilane hydrolysis and condensation effect of pH on reaction rates.
FIGURE 9—Base-catalyzed hydrolysis of alkoxysilanes.
≠
Si
OR
k1
B:
H2O
δ+
B:
H
O
H
δ−
Si
OR
Step 1
k-1
T.S.1
B:H+
HO
Si
−
δ
Si
k2
OR
k-2
Pentacoordinate
Intermediate
Si
OH
B:
≠
R
+
δ
H
O
Step 2
:B
T.S.2
ROH
!!
!
!
FIGURE 9—Base-catalyzed hydrolysis of alkoxysilanes.
Base-Catalyzed Condensation of Silanols A Head
R
Chojnowoski and Chrzczonowicz found that secondary and tertiary amines (piperidine, triethyl- amine, trik
n-butylamine) catalyzed the condensation of a series of 1dialkyl- or diarylsilanediols in aqueous dioxane.5
P primary
Si RandOH
P reactions
Si byNR
NH catalyzed the
Hattack
2
They proposed that
secondary amines
nucleophilic
on
2O
k1 of dialkylsilanediol
silicon which was followed by the rapid attack
on the silamine formed, as shown in
k-1
Figure
catalyzed
a general
mechanism.
R were
P 10.
SiTertiary
OHamineOH
P proceeded
Si by
NR
NH reactions
H O
OH base-catalyzed
!
2
2
k-1 of silanols.
FIGURE 10—Base-catalyzed condensation
R
OH
OH
R
k2
P
Si
OH
k-2
k2
OH
OH
!
NR
P
Si
OH
P
Si
2
k-2
FIGURE
10—Base-catalyzed
of silanols.
OH condensation
Acid-Catalyzed
Hydrolysis
of Alkoxysilanes
A Head OH
!!
!
NR2
P
Si
P
Si
O
SiR
O
OH
Si
P
OH NH
OH
R
P
NH
R
!6
The mechanism for the acid-catalyzed alkoxysilane hydrolysis reaction proposed by Osterholtz and Pohl
PAINT.ORG
|
51is a
shown in Figure
11. According to
and Pohl, the mechanism
for acid-catalyzed
hydrolysis
Acid-CatalyzedisHydrolysis
of Alkoxysilanes
A Osterholtz
Head
rapid equilibrium protonation of the substrate, followed by a bimolecular SN2-type displacement of the
leaving group by water.
NR2
Si
P
Si
OH
P
Si
k-2
OH
P
NROH
2
Si
!!NR
OH
!
P
Si
Si
P
2
Si
Si
k
OH2
OH
P
NH
POH Si
R
O
k-2
k2
Catalyzed Hydrolysis
of Alkoxysilanes A HeadOH
OH
!
P
O
OH
P
Si
OH
Si
P
O
R
Si
P
R
OH
NH
NH
R
echanism for the acid-catalyzed alkoxysilane hydrolysis
k-2 reaction proposed by Osterholtz and Pohl
wn in Figure Acid-Catalyzed
11. According to Hydrolysis
Osterholtz and
Pohl, the mechanism
for acid-catalyzed hydrolysis is
of Alkoxysilanes
A Head
Ra
OH
OH
2-type displacement
of the
equilibrium protonation of the substrate, followed by a bimolecular SNOH
g group by water.
The mechanism for the acid-catalyzed alkoxysilane hydrolysis reaction proposed by Osterholtz and Pohl
is shown in Figure 11. According to Osterholtz and Pohl, the mechanism for acid-catalyzed hydrolysis is a
cid-Catalyzed
Hydrolysis
of Alkoxysilanes
A Head
RE 11—Acid-catalyzed
hydrolysis
of alkoxysilanes.
rapid equilibrium
protonation
of the substrate, followed by a bimolecular SN2-type displacement of the
leaving group by water.
he mechanism for the acid-catalyzed alkoxysilane hydrolysis reaction
proposed by Osterholtz and Pohl
H
k1
shown in Figure 11. According to Osterholtz
and Pohl, the mechanism for acid-catalyzed hydrolysis is a
+
FIGURE
11—Acid-catalyzed
hydrolysis ofSi
alkoxysilanes.
Step displacement
1
Si
OR
OR
H
of the
pid equilibrium protonation of the substrate, followed by a bimolecular SN2-type
aving group by water.
k-1
H
!
Si
OR
H+
GURE 11—Acid-catalyzed hydrolysis
of alkoxysilanes.
Si
H
OR
Si
Si
Si
H
OR
k1
H+
k2
k-1 ≠
H+
H+
δ
δ
HOSi OR
Hk-2
OR
H
OR
Si
k2
H+ k
Si
-2
Step 2
HOR
H
OH
Si
δ
δ
HOSi OR
H 2O
H
OH
≠
H+
Step 1
OR
Step 1
≠
H+
H+
δ
δ
HOSi OR
H 2O
k2
Si
k-1
OR
H 2O
k1
H
OH
Si
HOR
HOR
Step 2
Step 2
k-2
H
OH
Si
k3
Si
!
k-3
Si
H
OH
H+
OH
Step 3
k3
Si
H+
OH
Step 3
The acid-catalyzed condensation reaction proceeds by an initial protonation of the silanol followed by an
-3 formation of water and regeneration of the acid catalyst as
FIGURE
11—Acid-catalyzed
hydrolysis
of kalkoxysilanes.
displacement
reaction
atk the
Si leading
to
N2
HSilanols
CatalyzedSCondensation
of
!
3A Head
depicted
in Figure 12.
+
! !!
Si
Si
OH
OH
H
Step 3
FIGURE 12—Acid-catalyzed
of silanols.
k-3condensation
Acid-Catalyzed Condensation
of Silanols A Head
!7
H
k1
P
Si
OH
O
cid-Catalyzed Condensation of Silanols A Head
H
P
OH2
!7
H 2O
k-1
H
OH
Si
OH
!7
k2
P
Si
OH
P
Si
OH2
P
Si
k-2
OH
OH
OH
O
H
Si
P
OH
k3
P
Si
O
Si
P
P
H 2O
O
Si
P
H3O+
k-3
H
!!
! FIGURE 12—Acid-catalyzed condensation of silanols.
! R'
R'
OH
!
Si
H 2O
OH
OH
OH
Tin Catalysis Mechanism A Head
As previously mentioned, tin catalysts are utilized in alkoxy, acetoxy, and oximinosilane systems. The tin
catalyst can be in the form of dioctyltin and dibutyltin diacetylacetonate and dibutyltin dilaurate (DBTDL).
Catalysis of organosilanes with tin compounds begins with hydrolysis of the tin compound. As shown in
Figure 13, the tin compound undergoes hydrolysis and forms an organotin hydroxide, which is the true
O organotin hydroxide reacts with an alkoxysilane
O
catalytic species.6 The
group to form an organotin
silanolate (Figure 14). The tin silanolate will react readily with alcohols and water to form the silanol
OH siloxane linkages,
(Figure
15).
The
organosilanolate
will
also
react
with
formed
silanol
groups
to produce
O
O
and the organotin hydroxide catalyst is regenerated (Figure 16).
!
R
Sn
R
FIGURE 13—Hydrolysis of tin catalysts.
R
Sn
R
H
O
! R'
R'
O
O
H
H
O
O
FIGURE 13—Hydrolysis
of tin catalysts.
GURE 14—Formation
of organotin
silanolates.
R'
52
O
| FEBRUARY 2016
!8
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 eighthour workday is 260 mg/m3. Methanol
exposure studies of a methoxysilane floor
adhesive based on an NIOSH method9
have been conducted that report 5400
mg/m3 emission of methanol during an
eight-hour period.10
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
proven inefficient for these reactions.
Catalyst Options
Tin compounds are very effective catalysts and they are commonly used for
crosslinking many of the organosilane
systems. The motivation for replacing
tin catalysts in these systems can be
for performance issues, but often the
motivation is due to regulatory issues.
Extensive studies were conducted
to identify compounds that could be
considered alternatives to tin catalysts
in organosilane systems crosslinked
with alkoxy, acetoxy, and oximinosilanes. The compounds evaluated in the
following experiments were a range of
metal compounds, acids, amines, and
combinations of each. In each of the
systems tested, a different compound
emerged as the most efficient.
In addition to the catalyst evaluation
in the following experiments, the issue
of methanol emission is addressed.
The control systems in each of the following experiments were catalyzed with
a tin catalyst. Non-tin catalysts included
in each of the experiments were K-KAT
670 (zinc complex), a proprietary
catalyst (Proprietary Compound), and
K-KAT XK-651 (bismuth carboxylate).
Compared to tin compounds, bismuth
and zinc compounds are more innocuous. They do not carry the environmental and biological restrictions of tin compounds. Extensive utilization of bismuth
organosilane systems. Experiment I is
and zinc compounds in the field of medR'R'
icine exemplifies
their non-toxic nature.R'R' based on a dimethoxymethylsilyl (DMS)
polyether
However, in general,OBi(III)
is known
O
OO polymer and Experiment II
addresses the issueOH
of
methanol generto have a high affinity for oxygen.11
OH
OO
OO
ation by using an organosilane based
Hydrolysis of bismuth carboxylate cataSn
RR Sn
RR consequentialRR on
Sn
RR
a diethoxysilyl
(DES) polyether
lysts is usually
aSn
negative
OO
HH
R'R' III is another
polymer. Experiment
effect of this affinity for oxygen.
K-KAT
OO
OO
OO
one-component
study, but the chemXK-651 is a bismuth
carboxylate
that
HH
istryHofHthe reactive polymers is based
addresses the hydrolysis
issue.
OO compared to tin
on a hydroxyl-terminated siloxane and
These R'
catalysts were
! ! R'
an oximinosilane crosslinker. Finally, a
catalysts in several different formulated
!!
FIGURE
14—Formation
organotin
silanolates.
FIGURE
14—Formation
ofof
organotin
silanolates.
R'R'
OO
OO
RR
Sn
Sn
RR
PP
OO
OCH
SiSi OCH
33
PP
RR
OCH
OCH
33
HH
CH
CH
3OH
3OH
OCH
SiSi OCH
33
OO
Sn
Sn
RR
! !FIGURE 14—Formation of organotin silanolates.
R'R'
! !
OO
FIGURE
15—Formation
silanol
groups.
FIGURE
15—Formation
ofof
silanol
groups.
R'R'
OO
OO
PP
OCH
SiSi OCH
33
RR
HH
2O
2O
PP
OO
OH
SiSi OH
RR
Sn
Sn
RR
OO
OCH
OCH
33
HH
Sn
Sn
RR
!! !!FIGURE 15—Formation of silanol groups.
! !
R'R'
OO
FIGURE
16—Formation
siloxane
groups.
FIGURE
16—Formation
ofof
siloxane
groups.
R'
O
O
P
Si
R
OCH3
P
O
Si
OH
OCH3
P
Si
O
OCH3
Si
P
OCH3
R
Sn
R
O
H
Sn
R
Experimental
Fully formulated single component
moisture-cure alkoxysilane systems
were used in the following experiments. The uncatalyzed one-component
formulations were stored in dispense
cartridges. Approximately 30 grams of
uncatalyzed material was dispensed
into a speed mixer container with a
caulk gun before addition of the catalyst. The material was mixed on a speed
mixer for 30 sec at 1500 rpm then 2 min
at 2200 rpm. Experiment IV followed
the same mixing process, but since
it was a two-component system, the
components were added to the mixing
container separately. An adjustable
doctor blade was used to apply approximately 3 mm of the blends onto a
paper substrate. The degree of dryness
was determined by using a Model 415
Drying Time Tester12 in accordance
with DIN 53 150. The dryness test
involved applying a force onto a paper
disk that covered a test site on the
casting for 60 sec. 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 25°C and 50% relative humidity.
Degree 1 of the DIN 53 150 method was
substituted with a glove test to determine touch dry. Table 2 defines the
rating system used.
Where tested, hardness of the castings was determined with a Shore A13
hardness tester after the castings were
allowed to cure under ambient conditions
for two weeks. Other mechanical properties were measured on an Instron14 tester
using dogbone-shaped samples cut from
the fully cured 3 mm thick castings.
Experiment I: Moisture-cure system based
on dimethoxymethylsilane (DMS) polymer
!!
FIGURE
16—Formation of siloxane groups.
!
!
!9 !9
two-component system based on a
silanol-terminated PDMS crosslinked
with an alkoxysilane is described in
Experiment IV.
R'
O
Tin EU Regulations A Head
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) 276/2010.7 A summary of these restrictions is presented in Table 1.8 With the stigma of
The control system in this experiment
was catalyzed with dioctyltin diacetylacetonate (DOTDAA) in a fully
PAINT.ORG |
53
SCOPE
SCOPE
SUBSTANCE
SUBSTANCE
REQUIREMENT
REQUIREMENT
EFFECTIVE
EFFECTIVE
Tri-substituted organostannic
compounds such as
Tributyltin (TBT) compounds
and 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 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
façades.
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 2015
Dioctyltin (DOT) compounds
%
DMS POLYMER15
32.8
PLASTICIZER
16.4
FILLER
39.3
TITANIUM DIOXIDE
6.6
THIXOTROPE
1.6
HALS
0.3
UVA
0.3
MOISTURE SCAVENGER
0.7
ADHESION PROMOTER
2.0
100.0
TABLE 3—Experiment I, DMS System Formulation
≤ 0.1 %
1 January 2012
formulated system based on a polyether
backbone DMS polymer. Catalyst levels
were derived from ladder studies. The
dosage of tin catalyst in the control
system was 0.6% DOTDAA. The tin
content of DOTDAA is approximately
21%. At 0.6%, the tin content in the formulated control system is approximately
TABLE 1—Summary of Organotin Requirements under European Regulation (EU) 276/2010 Amending Annex
0.12% which would not comply with EU
XVII of REACH.
REACHAAnnex
Methanol
Emission
HeadXVII, Entry No 20
regulations. The general formulation is
in Table 3.
If the R groups in Figures 2 and 4 are methyl, then the byproduct generated in the hydrolysis and
Dryness ratings of 3 mm thick castings
condensation reactions would be methanol. The European Agency for Safety and Health at Work (EUOSHA) directive 67/548/EEC, and the 25th updating of this directive (98/98/EC), has defined methanol as according to DIN 53 150 are in Table 4.
harmful with danger of very serious irreversible effects by inhalation, skin contact, and ingestion.
The DOTDAA and K-KAT 670 catalyzed
According to Commission Directive 2006/15/EC of February 7, 2006, the maximum allowable methanol
3
systems dried similarly with each achievexposure for an eight-hour workday is 260 mg/m . Methanol exposure studies of a methoxysilane floor
9
3
ing the highest degree of dryness (paper
adhesive based on an NIOSH method have been conducted that report 5400 mg/m emission of
10
methanol during an eight-hour period.
does not adhere to 20Kg load, no visible
1
TOUCH DRY, NO VISIBLE RESIDUE REMAINING ON RUBBER GLOVE
change to coated surface) in 6 h. The
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. Proprietary Compound and KKAT XK-651
2
PAPER
DOES inefficient
NOT ADHERE,
VISIBLE
CHANGE WITH 20 G LOAD
catalyzed systems cured significantly
Tin compounds
have
proven
forBUT
these
reactions.
slower. Differences in the performance of
the DOTDAA and K-KAT 670 catalyzed
3
PAPER
DOES
NOT
ADHERE,
BUT
VISIBLE
CHANGE
WITH
200
G
LOAD
Catalyst Options A Head
systems were not significant based on the
mechanical property results (Table 5).
Tin compounds
are
very
effective
catalysts
and
they
are
commonly
used
for
crosslinking
many
of
the
4
PAPER DOES NOT ADHERE, BUT VISIBLE CHANGE TO COATED SURFACE WITH 2 KG LOAD
organosilane systems. The motivation for replacing tin catalysts in these systems can be for performance The DOTDAA and K-KAT 670 castings
developed similar tensile stress (which can
11be associated with toughness), modulus
5
PAPER DOES NOT ADHERE, NO VISIBLE CHANGE TO COATED SURFACE WITH 2 KG LOAD
(elastic modulus) and strain (elongation).
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 2—Degree of Dryness (DIN 53 150)
54
| FEBRUARY 2016
Experiment II: Moisture-Cure System Based
on Diethoxysilane (DES) Polymer
Results of the DES polymer study
were very different compared to the
%
1
2
3
4
5
6
7
0.6% DOTDAA
0.8
2.3
3.5
4.0
4.5
5.0
5.5
DES POLYMER16
20.2
2.0% K-KAT 670
1.5
2.3
3.0
3.5
4.5
5.5
6.0
PLASTICIZER
23.1
2.0% PROPRIETARY COMPOUND
24+
24+
24+
24+
24+
24+
24+
FILLER
49.2
2.0% K-KAT XK-651
6+
6+
6+
6+
6+
6+
<23
TITANIUM DIOXIDE
3.3
ANTIOXIDANT
0.3
HALS
0.3
MOISTURE SCAVENGER
0.8
THIXOTROPE
1.4
ADHESION PROMOTER
1.5
TABLE 4—Experiment I, Degree of Dryness (h)
SHORE A
STRESS AT MAX, PSI
STRAIN AT MAX, %
MODULUS, PSI
0.6% DOTDAA
52
378
234
265
2.0% K-KAT 670
52
324
256
243
TABLE 5—Experiment I, Mechanical Properties. Cure: Two weeks ambient
DMS results. The basic uncatalyzed
DES formulation is given in Table 6.
DOTDAA was essentially not active in
this system. At 0.5% and 1.0% on total
formulation weight DOTDAA was not
effective. 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 h to achieve a
dryness rating of 7. Dryness ratings are
presented in Table 7.
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. The
Proprietary Compound and K-KAT
XK-651 catalyzed systems also cured
poorly. Dry times of the DES system catalyzed with K-KAT 670 were significantly
faster than the tin catalyzed systems. The
K-KAT 670 system developed dry times
that were comparable to the DMS system.
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 (Table 8).
1
2
3
4
5
6
7
0.5% DOTDAA
120
120
120
120
120
120
120
2.0% K-KAT 670
1.3
3.0
4.5
5.0
6.0
6.5
7.0
2.0% PROPRIETARY COMPOUND
7+
7+
7+
7+
7+
7+
7+
2.0% K-KAT XK-651
7+
7+
7+
7+
7+
7+
7+
TABLE 7—Experiment II, Degree of Dryness (h)
0.5% DOTDAAa
2.0% K-KAT 670
(a) one-month ambient cure
SHORE A
STRESS AT MAX, PSI
STRAIN AT MAX, %
MODULUS, PSI
22
93
267
85
48
291
608
103
TABLE 8—Experiment II, Mechanical Properties. Cure: Two weeks ambient.
100.0
TABLE 6—Experiment II, DES System Formulation
Experiment III: One-Component Moisture-Cure
System Based on Oximinosilane Polymer
The chemistry of another organosilane
crosslinking reaction that is effectively
accelerated with tin catalysts is based
on the reaction of hydroxyl terminated polydimethylsiloxane polymers
and oximinosilane crosslinker resins.
These systems can be supplied in a
one-component package.
The degree of dryness data in Table 9
exhibits efficient reaction acceleration
with DBTDL. With a dosage of only
0.1% catalyst on total formula weight,
the DBTDL system achieved the highest
degree of dryness (dryness degree = 7,
casting not effected by 20 Kg load) in 1.5
h. However, the system catalyzed with
2.0% K-KAT 670, which demonstrated
good activity in the one-component moisture cure alkoxysilane studies, required
nearly 4.5 h to achieve the same level of
dryness. The uncatalyzed control system
required 6 h to reach a dryness degree of
7. The system catalyzed with 2.0% K-KAT
XK-651 performed similarly to the K-KAT
670. Effective non-tin catalysis of the
crosslinking reaction was accomplished
with the Proprietary Compound. The
degree of dryness of the system catalyzed
with 0.5% of the Proprietary Compound
was essentially equal to the system catalyzed with 0.1% of DBTDL.
PAINT.ORG |
55
1
2
3
4
5
6
7
NO CATALYST
2.0
2.5
3.0
4.0
4.5
6.0
6.0
0.1% DBTDL
0.5
0.8
0.8
0.8
1.3
1.3
1.5
2.0% K-KAT 670
2.5
3.0
3.0
3.5
4.3
4.3
4.3
2.0% K-KAT XK-651
1.5
2.0
2.0
2.0
2.5
3.0
4.5
0.5% PROPRIETARY COMPOUND
0.3
1.0
1.0
1.0
1.3
1.5
1.5
1
2
3
4
5
6
7
NO CATALYST
24+
24+
24+
24+
24+
24+
24+
1.0% DBTDL
1.0
2.0
2.3
2.5
2.8
2.8
3.5
2.0% K-KAT 670
9+
9+
9+
9+
9+
9+
9+
2.0% PROPRIETARY COMPOUND
6+
6+
6+
6+
6+
6+
6+
2.0% COSCAT 83
6+
6+
6+
6+
6+
6+
6+
2.0% K-KAT XK-651
0.8
1.8
2.3
2.5
3.0
3.0
4.0
3.0% K-KAT XK-651
0.8
1.5
2.0
2.3
2.5
2.5
3.0
TABLE 9—Experiment III, Degree of Dryness (h)
TABLE 10—Experiment IV, Degree of Dryness (h)
Experiment IV: Two-Component
Silanol-Terminated PDMS with
Alkoxysilane Crosslinker
The following study of a two-component
organosilane formulation was based
on a silanol-terminated PDMS crosslinked with an alkoxysilane polymer.
The control catalyst in this study was
DBTDL. K-KAT 670 and the Proprietary
Compound, which were the most effective non-tin catalysts in the one-component
alkoxysilane and oximinosilane studies,
were included in this study. Also included
were two bismuth carboxylate catalysts,
Coscat 8317 and K-KAT XK-651.
As with the alkoxysilane studies, the
cure rate of the uncatalyzed system was
significantly slower than the optimized
systems with catalyst. In this case, the
K-KAT XK-651, which performed poorly
in the previous studies, provided the
best catalysis compared to the other
non-tin catalysts.
56
| FEBRUARY 2016
As previously mentioned, a potential drawback of bismuth catalysts is
limited hydrolytic stability. K-KAT
XK-651 was designed to provide
improved hydrolytic stability compared
to typical bismuth carboxylate catalysts. The results in Table 10 demonstrate a significant improvement in the
drying of the K-KAT XK-651 catalyzed
system compared to the Coscat 83 catalyzed system, which could be a manifestation of the improved hydrolytic
stability of K-KAT XK-651.
Experiment V: Two-component silanol terminated PDMS with acetoxysilane crosslinker
The following experiment was conducted with a simple clear system
based on a silanol terminated polydimethoxysiloxane18 with an acetoxysilane
crosslinker.19 The weight ratio of the
resins was 10:1. Films were cast with a
dry thickness of about 1.5 mils. Because
the films were relatively thin, dry times
were determined with circular drying
time recorders. 20 This method is based
on the circular scribe pattern on the
coating made by a teflon sphere stylus
that makes a complete 360° revolution
in 6 h. The three reported dry time
stages are Set to Touch—the time when
material stops flowing back into the
channel created by the stylus; Surface
Dry—when the stylus no longer leaves a
clear channel and begins to rupture the
dry upper layer of the film (also considered dust-free); and Through Dry—
when the stylus no longer ruptures or
dents the film. All films were aged at
25°C and 50% RH.
The system without catalyst was still
wet after 6 h. The system catalyzed
with 0.05% DBTDL reached the Set to
Touch stage in about 0.1 h, a Surface
Dry in about 0.25 h, and a Through
Dry in about 0.5 h. Each of the non-tin
SET TO
TOUCH
SURFACE
DRY
THROUGH
DRY
FILM
APPEARANCE
NO CATALYST
6+
6+
6+
CLEAR
0.05% DBTDL
0.1
0.25
0.5
CLEAR
2% K-KAT 670
0.3
0.9
2.0
CLOUDY
2% PROPRIETARY COMPOUND
0.25
2.5
3.0
CLOUDY
2% K-KAT XK-651
0.1
0.25
4.0
CLOUDY
2% K-KAT XK-648
0.2
0.6
1.4
CLEAR
TABLE 11—Experiment V, Circular Recorder Dry Times (h)
catalysts included in the previous
studies demonstrated some activity
in this study. However, some of the
catalysts developed compatibility
issues that were evident in the formulated blends in the pot and in the films.
Table 11 includes results of systems
catalyzed with the catalysts tested in
Experiments I through IV. An addition
to the catalyst list was K-KAT XK-648,
another zinc complex catalyst. The
catalyst dosages were 2% based on
formula weight.
The K-KAT 670 catalyzed system
developed relatively slow dry times
and appeared to have a compatibility
issue. The system catalyzed with the
proprietary catalyst was slower than
the DBTDL control, particularly in
Surface Dry. It also appeared to be
incompatible. K-KAT XK-651 provided
the best Set to Touch and Surface Dry
of the non-tin catalysts, but Through
Dry tailed off, possibly because of poor
compatibility that was evident in the
film appearance. The K-KAT XK-648
catalyzed system developed good dry
times and it appeared to be compatible.
Conclusion
A variety of chemistries fall under the
organosilane umbrella. Acceleration
of the curing process for many of the
chemistries is efficiently achieved with
tin catalysts. However, increasing
regulatory restrictions have formulators
searching for alternatives to tin
catalysts. This work has demonstrated
system specific catalysis in several
organosilane systems. K-KAT 670 was
the most efficient non-tin catalyst in a
one-component dimethoxymethylsilane
system which required acceleration of
both the hydrolysis and condensation
reactions. K-KAT XK-651 provided good
cure response in a system based on a twocomponent alkoxysilane crosslinked with
a silanol-terminated polydimethoxysilane
(PDMS). A proprietary compound was
the most effective non-tin catalyst
in a one-component oximinosilane
crosslinked system, and K-KAT XK-648
was the best tin alternative catalyst in
a silanol-terminated PDMS crosslinked
with an acetoxysilane.
Also addressed in this work was the
catalysis of a non-methanol emitting
moisture cure organosilane system
based on a diethoxysilane polymer.
In this case, K-KAT 670 catalyzed the
crosslinking reaction while tin catalysts were essentially not active even
at levels that doubled the maximum
tin concentrations allowable in current
EU regulations.
Acknowledgments
The authors would like to thank David
Switala and Matthew Gadman for their
contributions to this article.
References
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JOHN FLORIO ([email protected]) and RAVI
RAVICHANDRAN ([email protected]),
KING INDUSTRIES, Inc., USA
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