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RECENT ADVANCES WITH EXPANDABLE
GRAPHITE
IN
INTUMESCENT FLAME RETARDANT TECHNOLOGY
Kang Shen*1, Bernd Schilling2
1 Beijing
ADD-TECH Consulting Co., Ltd., Beijing
2 Nordmann, Rassmann GmbH,
Hamburg
ABSTRACT
Using expandable graphite as an
intumescent flame retardant in synthetic materials has
attracted more and more interests in both research and
industry circles in recent years.
Differ to normal expandable graphite,
which is mainly used for making expanded graphite sealing
material, the flame retardant grade expandable graphite has
its special properties.
This paper gives basic information of the
production of the expandable graphite, main types and
commercially available flame retardant grade expandable
graphite products and their special properties, like
adjustable special surface properties, SET (start expansion
temperature), expansion volume and expansion strength.
Nowadays,
expandable graphite had been used in plastics, rubbers
(elastomers), coatings, textiles and especially in polymeric
foams. To get perfect flame retardancy, using synergists, like
APP, EDAP, ZB is necessary. In some cases process condition as
well as equipment should also be modified.
With its
advantages of non-halogen, smoke reduction, non-heavy metals,
no immigration in base materials, expandable graphite had
become an important intumescent flame retardant.
Keywords:
Expandable graphite; Intumescent flame retardant
1. INTRODUCTION
Intumescent flame retardant systems
have been used in some materials for many years. Traditional
intumescent flame retardant system contains normally three
components that are the basic elements to form a char: source
of acid, gas and carbon. Commercially available such
intumescent flame retardant system today usually included
ammonium polyphosphate (acid donor), pentaerythritol(carbon
donor) and melamine(blowing agent). Material containing such
flame retardant system will form a char layer at the surface
during combustion to protect the material.
Recently,
expandable graphite as a single component intumescent flame
retardant had been studied and also used in some materials.
Such flame retardant grade expandable graphite has many
differences from its properties to production comparing with
traditional expandable graphite which is mainly used to make
expanded graphite and then further to make sealing
materials.
This
paper presents some recent advances regarding the expandable
graphite as an intumescent flame retardant, including its
production, properties and applications. Besides, the flame
retardancy of expandable graphite in some materials and how to
use the expandable graphite in industries were also
discussed.
2. Expandable graphite and its
production Expandable graphite is a graphite
intercalation compound (GIC). Compare with other pure carbon
material like diamond and C60, the special layer lattice
structure of flake graphite makes it possible to be
intercalated by other chemicals. Figure 1 shows different
lattice structure of flake graphite, diamond and C60.
Flake graphite
stems from nature graphite mine. The graphite ore is milled
and then floated. By this processing, the purity of flake
graphite can reach about 95%. To take out the remained organic
and inorganic impurities and get higher purity, we have to use
strong acid to soak the graphite and to bake the graphite
together with alkali at high temperature. Then after washing,
we can get the graphite with purity up to 99.5%.
With flake
graphite, the expandable graphite can be made through
intercalate acid (sulphuric acid, nitric acid, and acetic acid
are the commonly used acids) into the crystal layer of the
graphite. To introduce such acid into the graphite layer,
oxidants like H2O2, HNO3 or KMnO4 is necessary to be used. Or
we can use electrochemical method to prepare GIC. After
reaction, neutralizing, washing and drying are necessary.
For very long
time, expandable graphite was mainly used as an intermediate.
After expansion at 1000°C in oven, we get expanded graphite
which could be used to make graphite sheet and graphite
packing -- an important sealing material in industry. As an
intermediate product, expandable graphite had only a few
specifications. Most important items were particle size,
expansion volume, carbon content and volatiles. In some area,
low sulphur content was also very important. The storage time
was only guaranteed for 6 months. The pH value was lower than
4.
Since
1990's, expandable graphite has been used by more and more
amount as fire barrier material and flame retardants. For
flame retardant use, the specifications we need are much
complicated than normal expandable graphite. In the following
text, what we called the expandable graphite is just indicate
flame retardant grade expandable graphite. Nowadays high and
stable quality flame retardant grade expandable graphite
products are already commercially available. However, due to
the raw material was from natural mine, a good producer
normally should fix its raw material supply so as to avoid
quality changes from the raw material. In China, the main mine
area of flake graphite are in Shandong and Inner Mongolia.
People had already found that the flake graphite from these
two places had differences in properties, especially for the
impurities. It was found that the impurities of Inner Mongolia
graphite contained mainly soft ash - mica, but of Shandong
graphite contained mainly hard ash - quartz. It was found that
some properties of expandable graphite differ from different
producers. Even they use same sulphuric acid method to make
production.
In
the past time, people just concerned the expansion volume as
an important property. Now other properties like SET, become
more and more important to some applications. Many end-users
had already found such differences and have to select suitable
products for their special applications.
Table 1 shows some
important specifications of commercially available Nord-Min®
series expandable graphite. [1]
3. Requirements for flame retardant
application As a flame retardant, the basic
property of expandable graphite is the expansion, including
the behaviour of free expansion and expansion under pressure.
The start expansion temperature now became an important
parameter. People used to believe that the sulphuric acid
treated expandable graphite started expansion at 220°C and
nitric acid or organic acid treat expandable graphite started
expansion at 150°C.
The expansion was due to the decomposing
of the acid. Recent study found it was not really true. [2]
Depend on the reaction condition (include reaction time,
temperature, oxidant used and content of acid) and after
reaction treatment condition (inducing washing times and way
of drying), the SET usually can be from 170°C to 220°C for
sulphuric acid expandable graphite. So when you use expandable
graphite as a flame retardant in polymer, the process
temperature should be controlled to avoid its expansion during
the processing. Table 2 shows the start expansion temperature
of different expandable graphite products. 
*What we
called start expansion mean that the volume of expandable
graphite after expansion is not less than 150% of the original
volume. **This is the type similar as NORD-MIN® 249. In China, this grade name
(5095250) means the graphite (50mesh) has carbon content of
95% and expansion volume is 250. But from different producers,
the SET of their products changes from 175-190°C.
Besides these
thermal behaviours, for different applications, we met
different technical requirements for expandable graphite. For
polyurethane foam, neutral pH value and very clean surface was
necessary. Sometimes, we need to make special surface
treatment for the expandable graphite to avoid a big
increasing of the viscosity for the original PU formula system
when the expandable graphite was added.
If the foam will
be further processed at higher temperature, higher SET was
also needed. For polyolefin application, the particle size and
ash content as well as ash type were critical. We have to use
fine particle size graphite to keep the mechanical properties
of the material. Ash content should be controlled as low as
possible and the ash should be soft to avoid abrasion to the
processing machine, like extruder.
When expandable graphite was in flame
retardant coatings, the particle size should be fine and
surface should be treated. So as to be easy dispersed into the
coatings. Higher SET is necessary when expandable graphite was
used in bitumen roof sheet because some produces need to press
the sheets at higher temperature. Normally it is suggested to
control the SET at 210°C.
So we can see now that for flame retardant
application, there are many different technical requirements.
To use flame retardant grade expandable graphite is necessary
and using normal expandable graphite sometimes exists high
risks.
4. Flame retardancy
of expandable graphite and synergism
Expandable
graphite is halogen-free and works mainly in the condense
phase. For this reason, expandable graphite can strongly
reduced the smoke density in most cases. The expanded carbon
layer works as an insulating layer to reduce the heat
transfer. Tests showed that expandable graphite had good flame
retardancy comparing with other halogen-free flame retardants.
Recent research project "Flame retarded PP/Natural Fiber
Compounds "[3] showed that in polypropylene, compare with
other halogen-free flame retardants like ammonium
polyphosphate (APP), zinc borate(ZB), melamine cyanurate (MC)
etc, expandable graphite had a good effect in reducing the
burning rate and increasing the LOI. Figure 2 shows when the
flame retardants loading was 15%, the burning rate and LOI of
pure PP and its mixtures with different flame retardants. For
same system, a Cone calorimeter test showed that the PP with
expandable graphite had a much lower pHRR than PP with APP.
Figure 3 shows the pHRR of pure PP, PP with different loading
expandable graphite and PP with APP. Also from Cone
calorimeter test, it was found that expandable graphite could
reduce smoke greatly. Figure 4 shows the TSE of PP with 15% or
25% expandable graphite was much lower than the data of pure
PP or PP with 15% APP.
However, for some application, the
expanded carbon layers are too unstable, it is necessary to
use some other flame retardants to improve the flame retardant
property. Research showed that expandable graphite combined
with some halogen-free flame retardant additives, such as APP,
ZB, phosphorus-nitrogen containing compound NP28, and
microencapsulated red phosphorus, can apparently increase the
limiting oxygen index value and improve the flammability
properties of polyolefin blends. The data obtained from the
Cone calorimeter test indicate that the heat release rate and
effective heat of combustion decrease and the residues of
carbonaceous chars increase remarkably with addition of
expandable graphite and the other halogen-free flame retardant
additives. The thermal analysis data give the positive
evidence that the decreases of heat release rate and effective
heat of combustion values of the flame retarded polyolefin
materials are due to the increase of oxidative degradation
temperature and the decrease of oxidization heat [4].
5.
Applications
During the recent ten years, great
progress had been achieved regarding the application of
expandable graphite in flame retardant materials. Some main
applications are listed below.
5.1 Foam
Products
Due to its easy ignition and rapid fire
spread rate, reducing fire risk for polymeric foam products
had always been considered as a top priority. Now expandable
graphite had been used in PU foam. Such flame retarded PU foam
had been used as seat material in aircraft and other traffic
tools. It was also used as heat and sound insulation sheet for
car after special treatment.
In building industry, such flame retarded
PU foam has been used as sealing tape in windows and doors.
Besides used in PU foam, expandable graphite was also used at
EVA foam and now is of interest to be used in EPS foam.
5.2 Coating
Expandable
graphite had been found its application in intumescent flame
retarded coatings. Fine particle size expandable graphite had
been used in steel structure flame retarded coatings in many
countries.
5.3 Polyolefins
Expandable
graphite had already been used in PE, PP and other POs when
halogenated flame retardant was forbidden use, although up to
know after using expandable graphite the mechanical property
would be reduced more or less. Expandable graphite was mainly
used in Plastics /Metal (Al) composite board at the moment.
5.4 Bitumen Roof Sheets
Recently
expandable graphite had been used in bitumen roof sheets. For
this application the black colour was not a barrier for use.
Expandable graphite has very good flame retardancy to bitumen
roof material.
5.5 Other
applications
In China, using steel pipes as waste water
pipes in buildings are not allowed now. Plastics pipes now are
commonly used for rain water pipes and waste water pipes.
Anti-fire ring then is forced to be used by regulation when
the pipes pass through floors. Expandable graphite had been
used in such rings by large amount. Together with resin and
other additives, expandable graphite expanded during fire and
then sealed the space caused by its melting or burning of the
plastics pipes.
In other applications like fire-stop
putties, fire-stop pillows, expandable graphite had replaced
other intumescent agents and used widely.
In the back
adhesives of carpets, we could also find the existing of
expandable graphite.
6. New equipment for PU and other
applications
In some application areas, for example,
for PU foam industry, in order to use expandable graphite into
the original system, modification of the equipment was
necessary. Nowadays, company like Bayer/Hennecke as well as
Cannon had already found solutions to use expandable graphite
in PU foam production line by changing some components of the
equipment. Because normal expandable graphite has a particle
size of about 50mesh (0.3mm), if the expandable graphite
sprayed out with the polyol under high pressure, it would
damage the mixing header by strongly abrasion in one side. And
in other side, the expandable graphite flake will break into
very small pieces and lost its capability of expansion. So
make some modification to the mixing header is necessary and
in fact such work had already been done [5]. Besides,
expandable graphite should be dispersed into the polyol
homogenously, such a solid / liquid mixing system as well as a
suitable tube conveying system were also necessary to be
set.
7.
Conclusion and outlook
Due to the common efforts made by the
producers and end-users, expandable graphite, as a
halogen-free intumescent flame retardants, has been found a
lot of applications in industries. Today, expandable graphites
with special designed properties, so as to satisfy different
demands in industries, are already commercially available.
Many problems have been solved regarding product properties
and quality consistency. Comparing with other flame
retardants, the advantages of expandable graphite are:
- Non-toxic;
- No heavy metals;
- Halogen free;
- Not soluble in water and other
solvents;
- Smoke reduction and reduction of
toxic gas;
- Reduced HRR;
- Improved thermal conductivity;
- Synergism with other halogen-free
flame retardants;
- SET between 150 and 300 degree
C.
However, to use expandable graphite much widely,
there are still many works to be done. To change the black
colour, to reduce SET even lower and to get smaller particle
size but still with bigger expansion, etc, many new demands
had been put out from industries. On the other side, to work
together with equipments suppliers to improve the handling of
expandable graphite is still necessary for some applications.
References
[1] Technical data
sheet of Nord-Min®-Expandable Graphite. Nordmann, Rassmann
GmbH. May 2003.
[2] G.Camino,
S.Duquesne, etc.; Mechanism of Expandable Graphite Fire
Retardant Action in Polyurethane. 220th ACS meeting,
Washington DC, August 20-24, 2000.
[3]
Technical Report: Flame Retarded PP/Natural Fiber-Compound.
Prepared by Ostthueringische Materialpruefgesellschaft fuer
Textil und Kunststoffe mbH(TITK), in Rudolstadt. Published on
December 13, 2002.
[4] Rongcai Xie,
Baojun Qu; Synergistic effects of expandable graphite with
some halogen-free flame retardants in polyolefin blends. Polym
Degred Stab 2001, 71:375
[5] David
Reed; Graphite for FR foams. Urethanes Technology
August/September 1998: 9
Megunko Road
P.O. Box 349 Ashland, MA 01721
Tel: (508) 881-2220 Fax:(508) 881-1855
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