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KiON® ML85 Polysilazane

TB5: KiON
®
Polysilazanes - Reactivity with Epoxy Resins

Background:

KiON Polysilazanes are patented polymers which contain repeat units in which silicon and
nitrogen atoms are bonded in an alternating sequence. The majority of KiON Polysilazanes are
low viscosity liquids. KiON Polysilazanes all possess reactive Si-N functionality which enables
co-reaction with various organic resins such as epoxies, isocyanates, and phenols [3].

Basic Epoxy Reactivity:

Although silazanes are formally disilylamines, their reactivity with epoxies does not parallel that
of organic amines. Silazanes are not as basic as organic amines, and the reactivity of silazane-
based polymers with epoxy resins stems primarily from the polarity of the Si-N bond and its
reactivity with active hydroxyl functionality resulting from epoxy ring opening reactions. For
example, stoichiometric quantities of KiON Polysilazanes can be reacted with epoxy resins
based on the diglycidyl ether of Bisphenol "A" to give "cured" resin systems. Assuming
exhaustive reactivity for the Si-N bonds in a KiON polysilazane, the "amine" equivalent for these
polymers would be about 70 g/eq. This value, however, can only be considered a guideline for
formulation, since the fraction of Si-N bonds in a KiON Polysilazane which react with any given
epoxy resin varies from resin to resin. Ambient temperature shelf stability in excess of six
months for such dual component systems has been observed.

Epoxy resin cure using KiON polysilazanes can be achieved under pressure (autoclave curing)
by heating initially to 100°C for at least 2 hours (7°C / minute ramp) followed by a post-cure for
several hours (12 hours) at >165°C. Cure or postcure temperatures of as high as 260°C have
been used provided sufficient "pre-reaction" time at 100 °C is allowed. Compositions containing
high percentages of Bisphenol "A" diglycidyl ether epoxy resin were found to undergo phase
separation during cure, while compositions high in KiON polysilazanes [> 65 wt%] were rather
brittle. Sample formulations are shown below. [Please note, however, that none of these
systems have been optimized for stoichiometry or cure schedule. The observations described
should be used only as guidelines.] Our experience has shown that catalysts such as the N,N-
disubstituted anilines such as the Ancamine® catalysts [e.g. Ancamine® K54
tris(dimethylaminophenol)] or Lewis Acids are not desirable for the reaction of epoxy resins
with KiON polysilazanes, since they promote extensive foaming at temperatures lower than the
"pre-reaction" temperature (100 °C).

Epoxy Side Reactions:

The reactivity of the Si-N bond with active hydroxyl groups creates an inherent complication
when silazane-based polymers are used as hardeners for epoxy resins. As described above,
oxirane ring opening generates free hydroxyl groups which react with (Si-N) bonds to form Si-
O-C bonds in the reaction mixture. In the process of this reaction, terminal Si-NH 2 groups are
formed within the KiON Polysilazane. Such groups can then further react with either Si-NH-Si TB5
groups or other hydroxyl groups present in the reaction mixture to generate free ammonia, which
is evolved during the cure.

We have found, however, that these side reactions can be ameliorated in several ways: (1) by
allowing extended "pre-reaction" time at 100 °C before heating to cure temperature, (2) by
performing the cure in an autoclave, (3) by using a vinyl-containing KiON polysilazane (such as
KiON Ceraset Polysilazane 20) to promote vinyl crosslinking before cure [In this method a
peroxide initiator which has a decomposition half-life of less than 10 minutes at the cure
temperature (e.g. Elf Atochem's Lupersol® 256 or Lupersol® 231) is employed], or (4) by a
combination of the above.

By using such cure techniques we have generated continuous glass fiber-reinforced composite
laminates which demonstrate high hardness (115 Rockwell "M"), excellent flexural and tensile
strengths, and outstanding thermal stability (see following table). This resin system and cure
conditions are listed as dual component system A in the representative formulations section on
page 4.

S-2 Glass Fabric/Epoxy-KiON Polysilazane Hybrid Composite Properties
Test Conditions
Retained Flex Strength (ksi Mpa)
As fabricated
66 (465)
2 hour water boil
70 (490)
48 hours @ 480°F
31 (215)
24 hour water boil
0.7% weight gain
Temperature
Tensile Strength @ Temp. (ksi MPa)
25°C
127 (873)
150°C
117 (811)
300°C
110 (760)
400°C
103 (710)
500°C
42 (291)
Continuous use @ 30 ksi, 400°F
100 hours

The decomposition characteristics of such resin systems at progressively higher temperatures
demonstrate the synergism of incorporating an "inorganic" polysilazane "hardener" into a
conventional epoxy system. Thermal performance windows are extended toward higher
temperatures, decomposition pathways are gradual, and char formation is dramatically enhanced
(see attached TGA). In applications where cured resin systems having a higher degree of
toughness are desirable, straight-chain aliphatic epoxy diluents can be employed (C 8 / C 10
hydrocarbon substitution).

Page 2 of 6 TB5

The mechanical properties data for several unoptimized carbon fiber / KiON epoxy hybrid
polymer-reinforced composites are also shown below.

Short Beam Shear
Fiber
Fiber Vol.
Standard Resin (published values)
KiON hybrid resin
AS4 / 12k
62%
17.5 ksi (121 MPa)
7.4 ksi (51 MPa)
AS4 / 3k
62%
17.5 ksi (121 MPa)
7.9 ksi (54 MPa)
AS4 / 8H
62%
9.0 ksi (62 MPa)
4.5 ksi (31 MPa)

Tensile Strength
Fiber
Fiber Vol.
Standard Resin (published values)
KiON hybrid resin
AS4 / 12k
62%
225 ksi (1550 MPa)
192 ksi (1323 MPa)
AS4 / 3k
62%
225 ksi (1550 MPa)
167 ksi (1151 MPa)
AS4 / 8H
62%
120 ksi (827 MPa)
94 ksi (648 MPa)

Isocyanate Modifications:

Another approach has exploited the pronounced reactivity of the Si-N bond with organic
isocyanates. Isocyanates insert readily across the Si-N bonds of silazane-based polymers at
temperatures slightly above ambient to generate urea segments in the polymer chain. When such
urea segments contain active N-H bonds, rapid isomerization to the iso-urea configuration is
observed at temperatures above about 70°C (Equation 1).

m
Si N
H
H
Si N
H
C
R
N
O
Si
H
N
H
H
n
m
n-m
R
N
C
O
+
Si N
H
C
R
N
Si N
H
H
m
n-m
OH
70
O
C

Equation 1

This isomerization generates active hydroxyl functionality which can react with oxirane rings.
Thus, a three-component resin system consisting of KiON CERASET Polysilazane 20,
Bisphenol "A" diglycidyl ether, and isophorone diisocyanate reacts readily at 100°C with a post-
cure at > 165°C to yield a cured system without foaming.

Such three-component systems have limited shelf stability, since the reaction of the isocyanate
component with KiON polysilazanes advances at room temperature. Gelation will occur over
time. To circumvent these problems, especially when such compositions are being used in
coating applications, blocked isocyanates are used.

Page 3 of 6 TB5
Amine Modifications:

A second alternative involves the use of KiON Polysilazane-modified amine hardeners.
Conventional amine hardeners for epoxy resins can be heated in the presence of 20-25wt% of a
KiON polysilazane to prepare a KiON-modified hardener. In this method the organic amine
reacts with the KiON polysilazane at the Si-N bonds of the silazane-based polymers at
temperatures of about 90°C to incorporate the polysilazane into the amine system. Pronounced
foaming is observed at the reaction proceeds. When gas evolution subsides, the reaction is
complete. Epoxy cure can be effected by adding the modified-amine hardener to the epoxy resin
at proportionately higher amounts than recommended by the manufacturer (i.e. compensating for
the total mass of polysilazane added). While this method works very well for a variety of amine
hardeners, polyamide hardeners are not suited to this method.

Representative Formulations:

Listed below are three formulations with representative curing schedules (not optimized). We
have also cured epoxy compositions at higher temperatures (up to 260°C for 1-2 minutes for
coatings and thin films).

A) Dual Component System (can be used as a 1 part resin system)
60 grams KiON Ceraset Polysilazane 20 and 40 grams Bisphenol "A" diglycidyl ether
6 grams Lupersol® 256 2,5-dimethyl-2,5-di-(2-ethylhexanoyl-peroxy)hexane (Elf Atochem)
Blend all three components. Cure in air at 100°C for 2 hours (7° C / minute ramp from ambient
temperature). Post-cure at > 165°C for 12 hours. Cure is best achieved in an autoclave.

B) Two Part System (Part A + Part B)
Part A: 60 grams KiON Ceraset Polysilazane 20 and 10
grams Isophorone diisocyanate
Part B: 40