Download CYCLOALIPHATIC EPOXY RESINS Robert Kultzow* and Stephanie

CYCLOALIPHATIC EPOXY RESINS
Robert Kultzow* and Stephanie Foxhill
Huntsman Advanced Materials
Advanced Technology Center
8600 Gosling Road
The Woodlands, Texas
Presented at a meeting of the Thermoset Resin Formulators
Association at the Hyatt Regency Savannah in Savannah,
Georgia, September 10 through 11, 2007
This paper is presented by invitation of TRFA. It is publicly distributed upon
request by the TRFA to assist in the communication of information and
viewpoints relevant to the thermoset industry. The paper and its contents have
not been reviewed or evaluated by the TRFA and should not be construed as
having been adopted or endorsed by the TRFA
Abstract: Cycloaliphatic epoxy resins
are a class of materials of choice for a
wide variety of electrical, electronic and
structural applications.
Having fully
saturated molecular structures, they are
ideally suited for use in applications
requiring
resistance to ultraviolet
degradation and electrical arc-tracking.
This paper describes the commercially
available types of cycloaliphatic epoxy
resins, presenting their features and
applications. Physical property data for
a number of systems containing these
resins is provided.
This paper also
explores alternative epoxy resins in
formulations
where
cycloaliphatic
epoxies are typically used.
INTRODUCTION
Cycloaliphatic epoxy resins are a unique
class of materials that are characterized by
non-aromatic saturated rings in their
molecular structures.
There are two
commercial types of cycloaliphatic epoxy
resins in widespread use. Among their
most notable features are inherently low
viscosity, coupled with excellent weathering
and electrical performance.
Both
commercial cycloaliphatic resins are ideally
suited
for
applications
in
severe
environments such as near seacoasts and
in areas of high industrial pollution.
APPLICATIONS
An important application for cycloaliphatic
epoxy resins is for the fabrication of nonceramic outdoor electrical insulating
components. This would include electrical
insulators, bushings and switchgear parts
used for the transmission and distribution of
electrical power. Compared to porcelain,
components made with epoxy formulations
are lighter in weight, show greater impact
resistance, excellent electrical properties,
reduced maintenance and can be
fabricated economically into complex
shapes with embedded inserts2,4.
The
outstanding resistance to both ultraviolet
degradation and carbon tracking provided
by these resins are noteworthy1. They are
among the primary requirements for
electrical components used in severe
outdoor environments. The inherent low
viscosity of these resins enables them to be
formulated with higher levels of inorganic
fillers.
This enhances mechanical and
electrical track resistance for such
components.
Encapsulation of transformers, coils and
various electronic devices is another
important
area
of
application
for
cycloaliphatic epoxy resins.
Their
characteristics of low dielectric loss and
high electrical resistivity, up to or above
their glass transition temperatures provide
high performance and reliability in both AC
and DC circuitry. In such applications, low
viscosity enables facile penetration into
tightly-wound electrical coils.
Void-free
encapsulation is a pre-requisite in high
voltage electrical coils. It prevents failure
due to internal arcing and partial discharge.
The inherent low color characteristics
coupled with high resistance to yellowing
from ultra-violet light absorption makes the
cycloaliphatic epoxies useful in formulations
for such applications as light emitting
diodes.
Cycloaliphatic epoxy resin formulations are
used to fabricate many fiber-reinforced
structural components. Their low viscosity
allows for rapid fiber wet-out in commonly
used processes that include filament
winding, pultrusion and resin transfer
molding. Formulations incorporating these
resins can exhibit high glass transition
temperatures in the range of 200°C. These
are used to fabricate structural components
requiring high temperature service.
Cycloaliphatic epoxies show distinct
advantages over the glycidyl ether epoxy
materials in their ability to produce high
bond strengths on poorly cleaned and oily
surfaces.
A number of commercially
available adhesive formulations include
these materials for this benefit.
COMMERCIAL CYCLOALIPHATIC
EPOXIDES
An important and widely used cycloaliphatic
epoxy resin is the diglycidyl ester of
hexahydrophthalic acid.
This resin is
referred to Epoxy A throughout this paper.
Its structure is shown in Figure 1.
O
O
C
O
CH 2CH CH 2
C
O
CH 2CH CH 2
O
O
Figure 1
Epoxy A structure
Typical properties of this resin are given in
Table 1.
Table 1
Typical Properties
Color
Gardner 3 max.
EEW
164 - 172
Specific Gravity
1.20 – 1.25
Viscosity@ 25°C, cps.
700 - 900
Flash Point
374°F
Vapor Pressure, 20°C
<0.1 mbar
Another widely used cycloaliphatic epoxy
resin that has been readily available until
recent supply issues has the chemical
name of 3,4 epoxycyclohexylmethyl -3,4
epoxy-cyclohexane carboxylate. This resin
is referred to as Epoxy B throughout this
paper. Its chemical structure is illustrated in
Figure 2.
Figure 2.
Epoxy B structure
Typical properties of this material are
shown in Table 2.
Table 2
Typical Properties
Color
Gardner 1
EEW
131 - 143
Specific Gravity
1.14 – 1.18
Viscosity, 25°C
350 – 450 cps.
Flash Point
> 200°F
Vapor Pressure, 20°C
<0.1 mbar
Some cured physical properties of these
cycloaliphatic epoxy resins, as neat
systems (without filler) are given in Table 3.
Table 3
[Neat systems cured with HHPA + BDMA]
Property @ 25°C
Epoxy A
Epoxy B
Tensile strength (psi)
8,000
9,900
Tensile modulus (psi)
472,000
480,000
Elongation (%)
1.9
1.2
Flexural strength (psi)
19,300
12,900
Flexural modulus (psi)
490,000
440,000
Comp. strength (psi)
22,300
21,200
Tg (°C)
110 - 125
150 - 160
Dielec. constant, 60
3.3
3.3
Hz,
Diss. Factor, 60 Hz
0.005
0.005
Volume resist. (Ω/cm)
5 E16
5 E16
Dielectric Strength
476
Table 4
433
Two-component
casting
formulations
utilizing this epoxy resin that employ
saturated
acid
anhydride
curatives
commonly contain inorganic mineral fillers.
These fillers are used to increase thermal
conductivity and modulus while reducing
thermal expansion, reaction exotherm
during
gelation
and
cost.
The
recommended fillers for outdoor electrical
applications are silica and hydrated
alumina. Silane pre-treatment of these
fillers has been shown to significantly
improve retention of mechanical and
electrical properties on long term exposure
in outdoor conditions and in hostile
environments. Such silane coupling agents
provide a stronger bond between the resin
matrix and filler surfaces3,5. Some typical
properties of a pre-filled cycloaliphatic
epoxy system based on the Epoxy A
cycloaliphatic type containing 62% by
weight silanized silica filler are given in
Table 4.
Epoxy A / HHPA System
Containing 62% by weight silanized silica filler
Property @ 25°C
Typical
Values
Tensile strength, psi
Tensile modulus, psi
Flexural strength, psi
Flexural modulus, psi
Tg , °C
Dielectric constant, @ 60 Hz
Dissipation factor @ 60 Hz
Dielectric strength, 1/8”, v/mil
Time-to-track @ 2.5 kv
(ASTM-D 2303), minutes
12,000
1,500,000
18,000
1,450,000
110 - 120
3.5
0.005
450
>2000
Thermal endurance testing is used to
predict the long term stability of an
insulating material. Property changes such
as tensile or flexural strength are measured
after exposure to heat. The test times may
vary between 5,000 and 20,000 hours. A
thermal endurance profile, in the form of an
Arrhenius plot, is shown in Figure 3 for
anon-filled formulation utilizing the Epoxy A
type cycloaliphatic resin6. These tests were
performed in accordance with the IEC 216
Standard.
100000
Time at 50% Initial Strength (hours)
@ 1/8” (V/mil)
The most common curing agents for the
diglycidyl ester cycloaliphatic epoxy resin
include the carboxylic acid anhydrides and
acid catalytic types.
For outdoor
applications it is recommended to use fully
saturated curing agents in order to retain
both optimum uv light resistance and
electrical arc-tracking resistance. The most
common acid anhydrides of this type
include both hexahydrophthalic anhydride
and methylhexahydrophthalic anhydride. At
room ambient temperature, HHPA is a solid
but melts to a low very viscosity liquid at
35°-37°C. The MHHPA is a very low
viscosity liquid at room temperature having
a melting point of less than -15°C. Lewis
acid catalysts including both BF3 and BCl3
amine complexes are used to formulate
single component systems with this epoxy
resin. Such formulations exhibit excellent
latency at room temperatures with rapid
cure at elevated temperatures.
10000
1000
100
280
260
240
220
200
180
160
Temperature (°C)
Figure 3
Thermal Endurance Profile:
Epoxy A / HHPA System Non-Filled
140
120
The 3,4 epoxycyclohexylmethyl-3,4 epoxycyclohexane
carboxylate
resin
has
exceptionally low viscosity as well as very
low color. Similar to the diglycidyl ester
cycloaliphatic type, the most common
curing agents used with this epoxy are the
acid anhydrides and certain acid catalytic
types. This epoxy type is manufactured by
epoxidizing a diolefin with peracetic acid.
Since chlorine is not present in these raw
materials they are free of hydrolysable
chlorine. This characteristic is of great
importance in formulating high purity
electronic encapsulants.
Nadic-methyl
anhydride is frequently used in systems
demanding the highest glass transition
temperatures that can exceed 180°C, and
with certain high temperature cure
conditions, they can exceed 200°C.
However, NMA is generally less reactive
that other anhydrides and usually requires
longer cure cycles to develop full
properties. For such high Tg applications
this resin can either be used alone or in
combination with other epoxy resins for the
best balance of physical properties. Certain
Lewis acid type catalysts such as BF3amine complex types can be used with this
epoxy yielding systems with moderate
latency at room temperature coupled with a
very rapid elevated temperature cure.
In applications that require superior
electrical
properties
in
outdoor
environments, either of the cycloaliphatic
epoxy resins discussed in this paper can be
used if cured with fully saturated acid
anhydrides. In most of these applications,
glass transition temperatures in excess of
125°C are not required and generally not
desired. For such cases the Epoxy A resin
can be used without modification. However
if the type B resin is used for such
applications, it would commonly be modified
with flexibilizing additives, such as polyether
or polyester polyols.
This modification
would effectively reduce the Tg to the
desired range. Where outdoor electrical
performance is not a primary requirement
but low viscosity coupled with high Tg are of
paramount importance, the Epoxy B type
resin would be a material of choice. As an
alternative, this resin can be substituted for
blends of the Epoxy A with low viscosity
multi-functional epoxy resins such as 4glycidyloxy-N,N-diglycidyl aniline.
This
particular tri-functional epoxy is referred to
Epoxy C in this paper.
Its chemical
structure is illustrated in Figure 3. The Tg
values of several blends of Epoxy A and
Epoxy C, cured with two different
carboxcylic acid anhydrides are shown in
Table 5.
Figure 4
Epoxy C structure
Table 5
Cure: 2 h / 90°C + 1 h / 150°C + 1 h / 200°C
System
Formulations
Component
(pbw)
100
100
Epoxy B
75
50
75
Epoxy A
25
50
25
Epoxy C
110
105 120 MTHPA
115
110
NMA
0.5
0.5
0.5
0.5 0.5
1-m. imidazole
183 206
135 158 156
Tg by DSC, °C
50
50
130
0.5
181
CONCLUSIONS
Cycloaliphatic epoxy resins continue to be
used in a wide variety of electrical,
electronic and structural applications. Their
fully saturated molecular structures make
them ideal for use in applications where
protection
in
outdoor
and
severe
environments is needed In such cases they
are an attractive alternative to porcelain4.
There are two structurally different
commercially available cycloaliphatic epoxy
resins today. For many systems intended
for use in outdoor high voltage electrical
insulating components, either of these
epoxy resins can be used if properly
formulated.
Each of these cycloaliphatic
epoxy resins has a number of unique
characteristics favoring them for use in
certain
specialized
applications.
Formulations that utilize these versatile
epoxy resins can be tailored to meet the
requirements of many of these specialized
applications.
REFERENCES
1. U. Massen, C. Beisele, Cycloaliphatic
Epoxy Insulators – Experiences oner 30
Years, EFG-Fachtagung, Bad Nauheim,
Germany, (1999).
2. P. Mahonen, Outdoor Endurance of
Cycloaliphatic
Epoxy
Insulation,
International Symposium on Modern
Insulator Technologies, Coral Gables,
Florida, (1997).
3. U. Massen, Worldwide Outdoor
Experiences with Cycloaliphatic Epoxy
Insulators over 30 Years, Symposium on
Non-Ceramic
Insulator
Technology,
Singapore, (June 1996).
4. P.M. Vagle, Cycloaliphatic Epoxy
Outdoor Bushings / Insulators In a Heavily
Polluted Industrial Area – A Ultility’s View
Over a Decade, Georgia Power Company,
1996.
5. O. Margreve, B. Mistiaen, Silica-Filled
Cycloaliphatic Epoxy Resin Insulation,
Laboratory Research and Field Experience,
CIGRE 12 WG Instrument Transformers,
Chapter 2, Manufacture and Quality Tests,
GEC AlthomT&D Balteau S.A.
6. E. Hubler, B. Hanisch, Long Term
Experience with Cycloaliphatic Epoxy
Systems, SEE Conference, Paris, 1994