SAMPE Journal - May/June 2023 - 9

INTRODUCTION
Over the last few decades, there has been renewed
interest in the development of high-performance
thermosetting resins that can be processed below
200°C (400°F) and provide superior specific
strength values on par to high performance metallic
alloys.1
(Figure 1) As our technological demands
grow, the environments that these polymers
must readily withstand will shift from obscure
extremes hypothesized for future generations of
applications, into regularly experienced conditions
that will require materials engineering solutions.
Transoceanic passenger flights that complete their
journeys in less than an hour will need lightweight
engines that withstand supersonic air flows and
heats of combustion. Multiple-use space vehicles
that ferry astronauts back and forth between earth
and the International Space Station will demand
heat shields that endure extreme, high temperature
re-entries. High char retaining polymer solutions
stand out as materials that demonstrate necessary
performance and endurance in the face of these
challenging environments.
In industry, there are many polymer resin
classes to choose from, depending on the
processing requirements and end use. High
temperature thermosetting resins can be
categorized into four main classes: cyanate ester
(CE)2
, polyimide3
, bismaleimide (BMI)4
and
phthalonitrile (PN)5 resins. CE resins are plentiful,
easy to process, cure at reasonable temperatures
and can exhibit a high glass transition (Tg
) when
fully cured; however, the polymers can suffer from
issues due to their hydrolytic stability, depending
on how they are cured. They also exhibit an upper
temperature limit of around 400°C (750°F) with
moderate char yields.6
(e.g. PMR-15)7 have a high Tg
Thermosetting polyimides
, long-term thermal
stability, and good mechanical properties.
However, they are more difficult to process due to
the condensation cure chemistry, relative to other
resins, and absorb a fair amount of water (> 5%).8
Addition cured BMI resins show good structural,
thermal and electrical properties, but suffer from
difficulties in processability and have limited
long-term oxidative stability.9
PN resins combine
the excellent processability of CE resins, exhibit
low viscosities at cure temperatures, outstanding
thermal and mechanical properties, low water
absorption (<1.5%), and low flammability. All of
which the described properties are essential in
modern high temperature resin systems.
The US Naval Research Laboratory pioneered
the development of PN resins with the first
generation PN resins, which were synthesized
www. sampe.org
Figure 1. A comparison of the use temperatures and strength of high
temperature materials. Several polymer solutions illustrated in blue. Source:
https://www.grantadesign.com/ download/pstsmten05/.
from petroleum-based commodity chemicals.
They were prepared from the phenolate salts
of bisphenols (e.g. 4, 4'-biphenol, bisphenol A,
bisphenol A6F, etc.) and 4-nitrophthalonitrile.10-11
Although the resulting polymers show exceptional
properties, the PN resins exhibit a high melting
point (195-230°C) (380-450°F), which makes them
difficult to process into composite components.
At the time, the resins were costly to manufacture
and required high boiling polar aprotic solvents
(e.g., dimethylsulfoxide, N-methyl-2-pyrrolidone)
to prepreg. These processing limitations and high
cost led to the development of a 2nd
generation
PN resin that contain oligomeric aromatic ether
spacers between the terminal phthalonitrile
groups. These were synthesized via a two-step,
one-pot nucleophilic displacement reaction
between activated or unactivated dihaloaromatic
compounds and an appropriate bisphenol; followed
by end-capping with 4-nitrophthalonitrile12-23
. The
second generation PN resins addressed many of
the processing issues of the first generation, while
maintaining exceptional physical properties upon
polymerization. By the nature of the reaction
components and the subsequent reduction in
cost for 4-nitrophthalonitirle, these oligomeric
resins can now compete on cost with some
of the higher performance thermoplastic and
thermoset polymers (e.g. PEEKTM
appropriate scale.
or PEKKTM
) at the
MAY JUNE 2023
|
SAMPE JOURNAL |
9
https://www.grantadesign.com/download/pstsmten05/ http://www.sampe.org
SAMPE Journal - May/June 2023

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